Water-absorbent resin, hydropolymer, process for producing them, and uses of them

ABSTRACT

The present invention provides a production process by which a water-absorbent resin of excellent quality can be obtained at a low cost by reasonable steps in aqueous solution polymerization. The process for producing a water-absorbent resin comprises the step of polymerizing an aqueous solution of water-absorbent resin-forming monomers including acrylic acid and/or its sodium salt as major components, wherein: (1) the aqueous solution has a monomer component concentration of not less than 45 weight %; (2) the polymerization is carried out while water is evaporated so that the ratio (concentration ratio) between a solid component concentration in a hydropolymer as formed by the polymerization and a solid component concentration in the aqueous monomer solution will not be less than 1.10; and (3) the solid component concentration in the hydropolymer as formed by the polymerization is not more than 80 weight %.

BACKGROUND OF THE INVENTION

A. Technical Field

The present invention relates to: a process for producing awater-absorbent resin which is utilized favorably for various uses suchas sanitary articles (e.g. disposable diapers, sanitary napkins) andwater-retaining agents for soil, wherein the water-absorbent resin isproduced by polymerizing an aqueous solution of water-absorbentresin-forming monomers; products (hydropolymer and water-absorbentresin) from this process; and sanitary articles comprising thewater-absorbent resin.

B. Background Art

In recent years, water-absorbent resins are widely utilized for varioususes such as sanitary articles (e.g. disposable diapers, sanitarynapkins, adult incontinent products) and water-retaining agents for soiland are produced and consumed in large quantities.

Particularly in the uses for the sanitary articles (erg. disposablediapers, sanitary napkins, adult incontinent products), the tendency istoward increasing the amounts of water-absorbent resins and decreasingthe amounts of pulp fibers in order to render products thin, and thewater-absorbent resins are desired to have large absorption capacitiesunder loads, while the water-absorbent resins are used in so largequantities per sheet of the sanitary articles that the water-absorbentresins are desired to cost low. Therefore, in the production line of thewater-absorbent resins, it is desired to reduce energy consumption andwaste matter emission and to thereby establish a reasonable productionprocess.

Various polymerization processes have been attempted so far in which,for example, when aqueous solution polymerization of water-absorbentresin-forming monomers is carried out, a dried water-absorbent resin isobtained at one stroke by carrying out polymerization in a high monomerconcentration or initiating polymerization at a high temperature tothereby vaporize water by a heat of polymerization or heating, for thepurpose of rendering costs so low as to enhance the performance/costratio of the water-absorbent resin.

JP-A-071907/1983 (Arakawa Kagaku) and JP-A-018712/1984 (Arakawa Kagaku)disclose processes in which dry solids of water-absorbent resins areobtained at one stroke by polymerizing an aqueous acrylate salt solutionof a concentration higher than 55 weight %. U.S. Pat. No. 4,985,518(American Colloid) discloses a process in which a dry solid of awater-absorbent resin is obtained at one stroke by polymerizing anaqueous acrylate salt solution of a concentration higher than 30 weight%. JP-A-058208/1980 (Kitani) discloses a process in which polymerizationis carried out in the polymerization temperature range of 106 to 160° C.without any crosslinking agent, and in examples thereof a dry solidhaving a low water content is formed at the end of the polymerization.JP-A-318022/1989 (Mitsubishi Yuka) discloses a process in which apolymer is obtained in almost a dry state by polymerizing an aqueoussolution containing a monomer in a concentration of 45 to 80 weight %wherein the monomer has a neutralization ratio of 20 to 50 mol %.However, these processes have demerits in that the resultantwater-absorbent resins have high extractable contents for theirabsorption capacities.

In addition, JP-A-147512/1980 (Sumitomo Chemical Co., Ltd.),JP-A-147809/1981 (Sumitomo Chemical Co., Ltd.), JP-A-275607/1988 (SanyoKasei), and JP-A-275608/1988 (Sanyo Kasei) disclose processes in whichdried products of water-absorbent resins are obtained at one stroke bysupplying aqueous monomer solutions onto heated rotary drums and thenscraping off the resultant polymers therefrom. JP-A-165610/1989 (Rohmand Haas) also discloses almost the same process as the above in which asubstantially dry solid of a water-absorbent resin is obtained bysupplying an aqueous monomer solution onto a heated face. However, theseprocesses also have the demerits in that the resultant water-absorbentresins have high extractable contents for their absorption capacities.

In addition, JP-A-215801/1990 (Mitsubishi Yuka) discloses a process inwhich polymerization is carried out by spraying into a gas phase anaqueous monomer solution as heated by utilizing a heat of neutralizationof the monomer, but, as to this process, it is considered that thepolymerization is difficult to control, because the polymerization iscompleted in about 3 seconds.

The above prior arts were published in or before 1990, but have theirrespective demerits, therefore it seems that they are not actuallycarried out.

Thereafter published were arts as directed to enhancement of theperformance for the purpose of increasing the performance/cost ratio ofthe water-absorbent resin. JP-A-175319/1992 (Sanyo Kasei) andJP-A-181005/1999 (Nippon Shokubai Co., Ltd.) disclose attempts to obtainhigh-performance water-absorbent resins by initiating polymerization ata low temperature and mildly carrying out the polymerization whileremoving the generated heat to depress the peak temperature to nothigher than about 90° C. JP-A-228604/1999 (Nippon Shokubai Co., Ltd.)discloses an attempt to obtain a high-performance water-absorbent resinstill by initiating polymerization at a low temperature and mildlycarrying out the polymerization while removing the generated heat todepress the peak temperature to not higher than about 95° C. or tocontrol the increase of the solid component concentration within therange of 0.2 to 10 weight %. JP-A-067404/1997 (BASF) and U.S. Pat. No.6,187,828 (BASF) disclose a process in which polymerization is initiatedat a low temperature in a cylindrical polymerization reactor and carriedout adiabatically, but this process has demerits in that theconcentration of the aqueous monomer solution cannot be rendered high,because the heat removal is not carried out, and in that the residencetime is long (several hours). All these processes sacrifice theproductivity, therefore none of them can avoid high costs.

In addition, recently, “An Efficient Preparation Method forSuperabsorbent Polymers” (Chen, Zhao) was reported in the Journal ofApplied Polymer Science, Vol. 74, pp. 119-124 (1999). This proposes alow-cost polymerization process comprising the steps of placing anaqueous solution of a monomer concentration of 43.6% and an initiatorinto a stainless Petri dish, and then immersing this dish into a waterbath of 70° C. or 80° C. to carry out polymerization. However, thisprocess has not yet attained an industrially useful level.

In addition, JP-A-045812/1998 (Sekisui Chemical Products) discloses anattempt to inhibit bumping, promote emission of water vapor, and renderthe water content of the resultant gel low by adding short fibers to anaqueous monomer solution, but this attempt has the demerit of usingexpensive short fibers which do not contribute to the water absorption.

SUMMARY OF THE INVENTION

A. Object of the Invention

An object of the present invention is to provide a process by which awater-absorbent resin with excellent performance is produced at a lowcost and, specifically, to provide by reasonable steps the followingmaterials: a base polymer exhibiting a high absorption capacity withoutload and having a low extractable content; and a surface-crosslinkedwater-absorbent resin exhibiting a high absorption capacity under aload.

B. Disclosure of the invention

The present inventors diligently studied to achieve the above objectand, as a result, have completed the present invention by finding outthat, on the contrary to the conventionally accepted theory (which isthat, as is disclosed in the above JP-A-175319/1992 (Sanyo Kasei),JP-A-181005/1999 (Nippon Shokubai Co., Ltd.), and JP-A-228604/1999(Nippon Shokubai Co., Ltd.), a high-performance water-absorbent resin isobtained by initiating the polymerization at a low temperature and byrendering the peak temperature as low as possible by the heat removal),a high-performance water-absorbent resin can be obtained with highproductivity by a process in which a hydropolymer having a high solidcomponent concentration is obtained in a short time by setting thepolymerization initiation temperature for a high one and vaporizingwater at the boiling point of the resulting gel and which thereforeappears reckless in conventional views.

Herein, the term “hydropolymer” means a water-containing water-absorbentresin of which the solid component concentration is not more than 82weight %.

In addition, important for the production process according to thepresent invention is how a hydropolymer having a high solid componentconcentration of 55 to 82 weight % which is formable by polymerizationcan be disintegrated. In the case where the hydropolymer, which isformed by polymerizing an aqueous solution of water-absorbentresin-forming monomers, has a shape difficult to dry as it is, such asshapes of thick plates, blocks, and sheets, the hydropolymer is usuallydisintegrated and then subjected to the steps such as drying,pulverization, classification, and surface crosslinking, thus forming awater-absorbent resin product. In cases of acrylic acid (salt)-basedwater-absorbent resins, a hydropolymer having a solid componentconcentration of less than 55 weight % can easily be disintegrated withsuch as meat-chopper-type disintegrating machines. In addition, like adried hydropolymer, a hydropolymer having a solid componentconcentration of more than 82 weight % can easily be pulverized withsuch as conventional impact type pulverizing machines. However, ahydropolymer having a solid component concentration of 55 to 82 weight %is difficult to handle because of its properties and state, therefore anattempt to industrially disintegrate such a hydropolymer has not yetsucceeded.

For example, in Comparative Examples 1 and 2 as set forth in U.S. Pat.No. 4,703,067 (American Colloid), hydropolymers having solid componentconcentrations of 58% and 67% respectively are obtained, but this U.S.Pat. No. teaches that “they cannot be pulverized as they are, so theyneeded to be dried before being pulverized”. Thus, the disintegration inthe above solid component concentration range is avoided in theseComparative Examples.

JP-A-175319/1992 (Sanyo Kasei) discloses a gel-disintegrating machine asan example, but the polymerization is carried out in a monomerconcentration of 50 weight % at the highest, therefore this publicationdiscloses no example of disintegration of a hydropolymer having a solidcomponent concentration of not less than 55 weight %.

JP-A-119042/1998 (Nippon Shokubai Co., Ltd.), JP-A-188725/1999 (NipponShokubai Co., Ltd.), and JP-A-188726/1999 (Nippon Shokubai Co., Ltd.)disclose that a gel is disintegrated by shearing it with a fixed bladeand a rotary blade, but still these publications disclose no example ofdisintegration of a hydropolymer having a solid component concentrationof not less than 55 weight %.

JP-A-188727/1999 (Hatsuda, Miyake, Yano on Nippon Shokubai Co., Ltd.)discloses that a hydropolymer is disintegrated by shearing it byinterposing it between a pair of spiral rotary blades which are arrangedopposite to each other and fed at speeds different from each other. InExample 1 as set forth in this publication, a hydropolymer having awater content of 39 weight % is disintegrated, but this publication doesnot disclose any example in which the hydropolymer is disintegrated intoparticles of which the weight-average diameter is not larger than 100mm. Actually, the weight-average particle diameter of the disintegratedhydropolymer was larger than 100 mm.

Thus, the present inventors diligently studied about how a hydropolymerhaving a high solid component concentration of 55 to 82 weight % whichis formable by polymerization can be disintegrated. As a result, thepresent inventors have further found out that such a hydropolymer caneasily be divided into fine pieces with a specific disintegratingmachine.

That is to say, a process for producing a water-absorbent resin,according to the present invention, comprises the step of polymerizingan aqueous solution of water-absorbent resin-forming monomers includingacrylic acid and/or its sodium salt as major components, wherein:

-   -   (1) the aqueous solution has a monomer component concentration        of not less than 45 weight %;    -   (2) the polymerization is carried out while water is evaporated        so that the ratio (concentration ratio) between a solid        component concentration in a hydropolymer as formed by the        polymerization and a solid component concentration in the        aqueous monomer solution will not be less than 1.10; and    -   (3) the solid component concentration in the hydropolymer as        formed by the polymerization is not more than 80 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) the highest temperature during the polymerization is not        lower than 100° C.;    -   (2) the polymerization initiation temperature is not lower than        50° C.; and    -   (3) acrylic acid and/or water which evaporate during the        polymerization are collected and recycled.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) the polymerization initiation temperature is not lower than        50° C.;    -   (2) the solid component concentration in a hydropolymer as        formed by the polymerization is not more than 80 weight %; and    -   (3) the polymerization time is shorter than 3 minutes.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) at least one photoinitiator and at least one thermal        initiator are used together as polymerization initiators; and    -   (2) the highest temperature during the polymerization is not        lower than 105° C.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) at least one photoinitiator and at least one thermal        initiator are used together as polymerization initiators;    -   (2) the polymerization initiation temperature is not lower than        50° C.; and    -   (3) the aqueous solution has a monomer component concentration        of not less than 45 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its sodium salt as major components, wherein:

-   -   (1) the neutralization ratio of acrylic acid is not less than 50        mol %;    -   (2) the polymerization initiation temperature is not lower than        50° C.;    -   (3) the solid component concentration in a hydropolymer as        formed by the polymerization is not more than 80 weight %; and    -   (4) the polymerization time is shorter than 3 minutes.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) the polymerization initiation temperature is not lower than        50° C.;    -   (2) the aqueous solution has a monomer component concentration        of not less than 45 weight %; and    -   (3) the polymerization temperature-rising ratio is not more than        0.30.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its sodium salt as major components, wherein:

-   -   (1) the polymerization initiation temperature is not lower than        50° C.;    -   (2) the aqueous solution has a monomer component concentration        of not less than 45 weight %; and    -   (3) the highest temperature during the polymerization is not        higher than 140° C.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its sodium salt as major components, wherein:

-   -   (1) the aqueous solution has a monomer component concentration        of not less than 45 weight %;    -   (2) the neutralization ratio of acrylic acid is not less than 50        mol %;    -   (3) the polymerization initiation temperature is not lower than        50° C.; and    -   (4) the difference between the polymerization initiation        temperature and the highest temperature during the        polymerization is not more than 70° C.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein the polymerizationproceeds under extension force.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components,

wherein the process further comprises the step of disintegrating ahydropolymer into particles of which the weight-average diameter is notlarger than 100 mm wherein the hydropolymer is formed by thepolymerization and has a solid component concentration in the range of55 to 82 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components,

wherein the process further comprises the step of disintegrating ahydropolymer with a disintegrating machine having a screen wherein thehydropolymer is formed by the polymerization and has a solid componentconcentration in the range of 55 to 82 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components,

wherein the process further comprises the step of disintegrating ahydropolymer with a disintegrating machine so that the ratio of theincrease of the solid component concentration during the disintegrationmay not be less than 2 points wherein the hydropolymer is formed by thepolymerization and has a solid component concentration in the range of55 to 82 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components,

wherein the process further comprises the step of disintegrating ahydropolymer with a disintegrating machine while passing a gas throughthe disintegrating machine wherein the hydropolymer is formed by thepolymerization and has a solid component concentration in the range of55 to 82 weight %.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components,

wherein the process further comprises the step of surface-crosslinking aparticulate hydropolymer which is obtained by disintegrating ahydropolymer resultant from the polymerization and has a solid componentconcentration in the range of 55 to 82 weight %, a residual monomercontent of not more than 1,000 ppm, and a weight-average particlediameter of not larger than 3 mm.

Another process for producing a water-absorbent resin, according to thepresent invention, comprises the step of polymerizing an aqueoussolution of water-absorbent resin-forming monomers including acrylicacid and/or its salt as major components, wherein:

-   -   (1) the polymerization step produces a hydropolymer having a        solid component concentration in the range of 55 to 82 weight %;    -   and wherein the process further comprises the following steps:    -   (2) a disintegration step for disintegrating the hydropolymer,        which has a solid component concentration in the range of 55 to        82 weight %, into particles of which the weight-average diameter        is not larger than 10 mm; and    -   (3) a drying step for increasing the solid component        concentration in the disintegrated hydropolymer by not less than        3%.

A water-absorbent resin, according to the present invention, is obtainedby a process including the step of polymerizing an aqueous solution ofwater-absorbent resin-forming monomers including acrylic acid and/or itssalt as major components, and has the following properties:

-   -   (1) 20 (g/g)≦absorption capacity without load (GV)≦60 (g/g);    -   (2) absorption capacity under a load (AAP)≧20 (g/g); and    -   (3) absorption capacity without load (GV)×solubilization residue        ratio (%)≦1,200 ((g/g)%).

A hydropolymer, according to the present invention, is a disintegratedhydropolymer which is obtained when producing a water-absorbent resin bya process including the step of polymerizing an aqueous solution ofwater-absorbent resin-forming monomers including acrylic acid and/or itssalt as major components, and has a solid component concentration in therange of 55 to 82 weight %, a residual monomer content of not more than1,000 ppm, and a weight-average particle diameter of not larger than 3mm.

A sanitary article, according to the present invention, comprises atleast one member selected from the group consisting of thewater-absorbent resins obtained by the above production processesaccording to the present invention and the above water-absorbent resinaccording to the present invention.

These and other objects and the advantages of the present invention willbe more fully apparent from the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which illustrates relations between polymerizationreaction temperature and time in Examples 3 and 7 to 10, wherein thetime zero is the initiation time of irradiation of light.

FIG. 2 is a side view photograph of the hydropolymer as obtained inExample 1.

FIG. 3 is a side view photograph of the hydropolymer as obtained inExample 3.

FIG. 4 is a top view photograph of the hydropolymer as obtained inExample 3.

FIG. 5 is a bottom view photograph of the hydropolymer as obtained inExample 3.

FIG. 6 is a side view photograph of the hydropolymer as obtained inExample 5.

FIG. 7 is a side view photograph of the hydropolymer as obtained inExample 7.

FIG. 8 is a photograph of a gel as formed by cutting off a portion ofthe hydropolymer resultant from Example 7 and then swelling it with tapwater.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, modes for carrying out the present invention are explainedin detail.

Examples of the water-absorbent resin-forming monomers, as used in thepresent invention, include anionic unsaturated monomers, such as(meth)acrylic acid, (anhydrous) maleic acid, itaconic acid, cinnamicacid, vinylsulfonic acid, allyltoluenesulfonic acid,vinyltoluenesulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonicacid, and 2-hydroxyethyl (meth)acryloyl phosphate, and their salts;mercaptan-group-containing unsaturated monomers;phenolic-hydroxyl-group-containing unsaturated monomers;amide-group-containing unsaturated monomers such as (meth)acrylamide,N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; andamino-group-containing unsaturated monomers such asN,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide. Thesemonomers may be used either alone respectively or fitly in combinationswith each other. However, for performance and costs of the resultingwater-absorbent resin, it is necessary to use acrylic acid and/or itssalt (e.g. salts of such as sodium, lithium, potassium, ammonium, andamines, wherein the sodium salt is particularly favorable for costs) asmajor components. The acrylic acid and/or its salt is used in a ratio offavorably not less than 70 mol %, more favorably not less than 80 mol %,still more favorably not less than 90 mol %, particularly favorably notless than 95 mol %, to the entirety of the monomer components.

Besides the unsaturated monomer components such as acrylic acid and/orits salt and internal-crosslinking agents, other additives such aspolymerization initiators as mentioned below are also included in thesolid components in the aqueous monomer solution as referred to herein.

Conventional internal-crosslinking agents can be used as the aboveinternal-crosslinking agents. Specific examples thereof include thosewhich are disclosed on page 4 of JP-A-182750/1998. Theseinternal-crosslinking agents may be used either alone respectively or incombinations with each other in consideration of their reactivity.Particularly, it is favorable that a compound with at least twopolymerizable unsaturated groups is essentially used. The amount of theabove internal-crosslinking agent, as used, is determinable fitly forthe properties of the aimed water-absorbent resin.

The concentration of the water-absorbent resin-forming monomers is notespecially limited, but is favorably not less than 30 weight %, morefavorably not less than 35 weight %, still more favorably not less than40 weight %, yet still more favorably not less than 45 weight %, yetstill more favorably not less than 50 weight %, yet still more favorablynot less than 55 weight yet still more favorably in the range of 30 to70 weight %, yet still more favorably in the range of 35 to 60 weight %,yet still more favorably in the range of 40 to 60 weight %. In the casewhere the concentration is less than 30 weight %, the productivity islow. In the case where the concentration is more than 70 weight %, theabsorption capacity is low.

The neutralization ratio of the acid-group-containing monomer is notespecially limited, but is favorably not less than 50 mol %, morefavorably in the range of 50 to 80 mol % (but not including 80 mol %),still more favorably in the range of 55 to 78 mol %, most favorably inthe range of 60 to 75 mol %, also considering that the neutralizationsubsequent to the polymerization is not needed for uses having apossibility of contact with human bodies, such as sanitary articles.

In the case where acrylic acid is used in the form neutralized with analkali, it is favorable that a heat of neutralization and/or a heat ofdissolution (of acrylic acid and an alkali) is effectively utilized forraising the temperature of the aqueous monomer solution. In a favorablemode for carrying out the present invention, the polymerization isinitiated by adding a crosslinking agent and an initiator to the aqueousmonomer solution as heated by the neutralization in an adiabatic stateor, as is mentioned below, the heat of neutralization and/or the heat ofdissolution (of acrylic acid and the alkali) are utilized for removal ofdissolved oxygen.

When the polymerization is carried out, the following materials may beadded to the reaction system: hydrophilic polymers such as starch, itsderivatives, cellulose, its derivatives, poly(vinyl alcohol),poly(acrylic acid) (or its salts), and crosslinked polymers ofpoly(acrylic acid) (or its salts); chain transfer agents such ashypophosphorous acid (or its salts); and chelating agents.

With regard to the method for polymerizing the above monomer components,there is no especial limitation if it is aqueous solutionpolymerization. For example, the present invention can be carried out bystatic polymerization in which the aqueous monomer solution ispolymerized in a static state or by agitation polymerization in whichthe aqueous monomer solution is polymerized in an agitation apparatus.

In the static polymerization method, an endless belt is favorably used.Favorable examples of the belt include resin-made or rubber-made beltssuch that the heat of polymerization is difficult to escape from theface (of the belts) contacting the materials.

In the agitation polymerization method, single-shaft agitators are alsoavailable, but multiple-shaft agitators are favorable.

In cases of the radical aqueous solution polymerization, conventionally,dissolved oxygen which hinders the polymerization is removed from theaqueous solution by blowing an inert gas into the aqueous solution ordeaerating the aqueous solution under reduced pressure before thepolymerization initiator is added. In the actual circumstances, however,facilities and operation costs are needed therefor. In a favorable modefor carrying out the present invention, the operation for removing thedissolved oxygen is carried out by utilizing the aforementioned heat ofneutralization and/or the aforementioned heat of dissolution (of acrylicacid and the alkali) for raising the temperature of the aqueous monomersolution and thereby volatilizing the dissolved oxygen.

In a more favorable mode for carrying out the present invention, rawmaterials (such as acrylic acid, an aqueous alkali solution, and water)for the aqueous monomer solution can be heated by neutralization withoutbeing deoxidized beforehand, so that the dissolved oxygen content can beadjusted to favorably not more than 4 ppm, more favorably not more than2 ppm, most favorably not more than 1 ppm, of the aqueous monomersolution, and then the aqueous monomer solution can be subjected to thepolymerization intactly without deoxidation operation.

In addition, it is also favorable that a part or all of the rawmaterials (such as acrylic acid, an aqueous alkali solution, and water)for the aqueous monomer solution are beforehand partially deoxidized andthen further deoxidized by raising the temperature by the neutralizationand. In addition, in the case where the polymerization is initiated at ahigh temperature of not lower than 80° C. by line-mixing acrylic acidand an alkali to make neutralization and further line-mixing apolymerization initiator, it is favorable for inhibition of thepolymerization initiation in lines that the raw materials (such asacrylic acid, an aqueous alkali solution, and water) are not deoxidizedbeforehand.

The polymerization is carried out usually under normal pressure, but itis also a favorable mode that the polymerization is carried out whilewater is distilled off under reduced pressure in order to lower theboiling temperature of the polymerization system. More favorably forsuch as easiness of the operation, the polymerization is carried outunder normal pressure.

The increase of the neutralization ratio during the polymerization isnot especially limited, but is favorably not less than 2 points, morefavorably not less than 3 points, still more favorably not less than 4points. Even if the increase of the neutralization ratio is 0, there isno especial problem. However, the increase of the neutralization ratioof not less than 2 points has the advantage of enhancing the propertiesof the resultant polymers (hydropolymer, base polymer, water-absorbentresin).

The polymerization initiator, as used in the present invention, is notespecially limited, but usable examples thereof include: thermalinitiators (e.g. persulfates such as sodium persulfate, potassiumpersulfate, and ammonium persulfate; peroxides such as hydrogenperoxide, t-butyl peroxide, and methyl ethyl ketone peroxide; azocompounds such as azonitrile compounds, azoamidine compounds, cyclicazoamidine compounds, azoamide compounds, alkylazo compounds,2,2′-azobis(2-amidinopropane) dihydrochloride, and2,2′-azobis(2-(2-imidazolin-2-yl)propane) dihydrochloride); andphotoinitiators (e.g. benzoin derivatives, benzyl derivatives,acetophenone derivatives, benzophenone derivatives, azo compounds).Considering the costs and the ability to reduce the residual monomercontent, the persulfates are favorable. In addition, the use of the atleast one photoinitiator and ultraviolet rays is also a favorablemethod. More favorably, the at least one thermal initiator and the atleast one photoinitiator are used together.

It is favorable to beforehand render the monomer temperature high. Thereason therefor is that such a way facilitates the aforementionedremoval of dissolved oxygen and further makes it possible to immediatelyrealize the below-mentioned favorable polymerization initiationtemperature. Such a monomer temperature is not especially limited, butis usually not lower than 50° C., favorably not lower than 60° C., morefavorably not lower than 70° C., still more favorably not lower than 80°C., yet still more favorably not lower than 90° C., yet still morefavorably in the range of 80 to 105° C., most favorably in the range of90 to 100° C. In the case where the monomer temperature is lower than50° C., the induction period and the polymerization time lengthen somuch as to deteriorate not only the productivity but also the propertiesof the resulting water-absorbent resin. Incidentally, the polymerizationtime is a period of time of from completion of the polymerizationinitiation conditions following the charge of the aqueous monomersolution into a polymerization reactor till attainment to the peaktemperature.

The polymerization initiation temperature is usually not lower than 50°C., favorably not lower than 60° C., more favorably not lower than 70°C., still more favorably not lower than 80° C., yet still more favorablynot lower than 90° C., yet still more favorably in the range of 80 to105° C., most favorably in the range of 90 to 100° C. In the case wherethe polymerization initiation temperature is lower than 50° C., theinduction period and the polymerization time lengthen so much as todeteriorate not only the productivity but also the properties of theresulting water-absorbent resin. In the case where the polymerizationinitiation temperature is higher than 105° C., the foaming or extensionmight not sufficiently occur. The polymerization initiation temperaturecan be observed from such as white turbidity, increase of viscosity, andrise of temperature of the aqueous monomer solution.

Incidentally, as is aforementioned, the heat of neutralization of theaqueous monomer solution and/or the heat of dissolution (of acrylic acidand the alkali) are favorably utilized for securing the temperature ofthis aqueous monomer solution to cause the polymerization initiation.

The highest temperature during the polymerization is not especiallylimited, but is favorably not higher than 150° C., more favorably nothigher than 140° C., still more favorably not higher than 130° C., yetstill more favorably not higher than 120° C., yet still more favorablynot higher than 115° C. In the case where the highest temperature ishigher than 150° C., there are disadvantages in that the properties ofthe resultant polymers (hydropolymer, base polymer, water-absorbentresin) are greatly deteriorated.

In the present invention, the difference between the polymerizationinitiation temperature and the highest temperature during thepolymerization is favorably not more than 70° C., more favorably notmore than 60° C., still more favorably not more than 50° C., yet stillmore favorably not more than 40° C., yet still more favorably not morethan 30° C., most favorably not more than 25° C. In the case where thedifference between the polymerization initiation temperature and thehighest temperature during the polymerization is more than 70° C., thereare disadvantages in that the properties of the resultant polymers(hydropolymer, base polymer, water-absorbent resin) are deteriorated.

The polymerization time is not especially limited, but is favorably notlonger than 5 minutes, more favorably not longer than 3 minutes, stillmore favorably shorter than 3 minutes, yet still more favorably notlonger than 2 minutes, yet still more favorably not longer than 1minute. In the case where the polymerization time is longer than 5minutes, there are disadvantages in that the productivity of theresultant polymers (hydropolymer, base polymer, water-absorbent resin)is deteriorated.

In a favorable example of the polymerization process according to thepresent invention, after the initiation of the polymerization, thetemperature of the system rapidly rises to reach the boiling point inthe stage of a low polymerization conversion, for example, of 10 to 20mol %, at which the system emits water vapor and increases its solidcomponent concentration, while the polymerization proceeds. Therefore,it is desirable that the heat radiation from portions (of thepolymerization reactor) contacting the materials is suppressed as muchas possible, and examples of qualities of materials as favorably usedtherefor include such that portions (made of such as resin, rubber, andstainless steel) noncontacting the materials are covered with heatinsulators or heated with a jacket. The water vapor as emitted from thesystem might contain monomers. In such a case, therefore, the watervapor is desired to be recovered and recycled. Particularly, it isfavorable that acrylic acid and/or water which evaporate during thepolymerization are collected and recycled. The recovery ratio of theacrylic acid is favorably not less than 1%, more favorably not less than2%, still more favorably not less than 3%, based on the whole weight(before neutralization) of the acrylic acid as used.

In addition, in the process according to the present invention, thepolymerization is carried out at a high temperature from the initiationof the polymerization, and it is inferred that such polymerization is acause of high performance. As to polymerization under normal pressure, afavorable mode thereof is a polymerization such that: when thepolymerization conversion has reached 40 mol %, the temperature of thesystem has already been a temperature of not lower than 100° C., andeven when the polymerization conversion has reached 50 mol %, thetemperature of the system is still a temperature of not lower than 100°C. And a more favorable mode is a polymerization such that: when thepolymerization conversion has reached 30 mol %, the temperature of thesystem has already been a temperature of not lower than 100° C., andeven when the polymerization conversion has reached 50 mol %, thetemperature of the system is still a temperature of not lower than 100°C. And the most favorable mode is a polymerization such that: when thepolymerization conversion has reached 20 mol %, the temperature of thesystem has already been a temperature of not lower than 100° C., andeven when the polymerization conversion has reached 50 mol %, thetemperature of the system is still a temperature of not lower than 100°C. In cases of polymerization under reduced pressure, likewise, afavorable mode thereof is a polymerization such that: when thepolymerization conversion has reached 40 mol %, the temperature of thesystem has already been the boiling temperature, and even when thepolymerization conversion has reached 50 mol %, the temperature of thesystem is still the boiling temperature. And a more favorable mode is apolymerization such that: when the polymerization conversion has reached30 mol %, the temperature of the system has already been the boilingtemperature, and even when the polymerization conversion has reached 50mol %, the temperature of the system is still the boiling temperature.And the most favorable mode is a polymerization such that: when thepolymerization conversion has reached 20 mol %, the temperature of thesystem has already been the boiling temperature, and even when thepolymerization conversion has reached 50 mol %, the temperature of thesystem is still the boiling temperature.

In this way, the system reaches a high temperature when thepolymerization conversion is still low. Therefore, the time as neededfor the polymerization is also short, and it is usual for thepolymerization to end in not longer than 10 minutes, favorably notlonger than 5 minutes. Incidentally, the time as needed for thepolymerization is a period of time of from charge of a polymerizationreactor with the aqueous monomer solution, to which the polymerizationinitiator has been added, till discharge of the resultant hydropolymerfrom the polymerization reactor.

In the present invention, it is desirable that the polymerization iscarried out while water is evaporated so that the ratio (concentrationratio) between a solid component concentration in the hydropolymer asformed by the polymerization and a solid component concentration in theaqueous monomer solution will favorably not be less than 1.10, morefavorably not be less than 1.15, still more favorably not be less than1.20, yet still more favorably not be less than 1.25. In the case wherethe concentration ratio is less than 1.10, the utilization of the heatof polymerization cannot be said to be sufficient. Incidentally, thesolid components in the aqueous monomer solution are monomers and otheradditives, and do not include water or solvents.

In the present invention, the ratio of temperature rising during thepolymerization, namely, the ratio of ΔT (difference between the highesttemperature during the polymerization and the polymerization initiationtemperature; ° C.), as observed for the polymerization system, totheoretical ΔT (° C.), (the details of the calculation method for thisratio are described in the portion hereof under the heading of “DETAILEDDESCRIPTION OF THE PREFERRED EMBODIMENTS”) is favorably not more than0.30, more favorably not more than 0.25, still more favorably not morethan 0.20. In the case where the ratio of temperature rising during thepolymerization is more than 0.30, the utilization of the heat ofpolymerization for the evaporation of water is so insufficient thatthere are disadvantages in that the properties of the resultant polymers(hydropolymer, base polymer, water-absorbent resin) are deteriorated.

The hydropolymer, as obtained by the above polymerization, has a solidcomponent concentration of favorably not more than 82 weight %, morefavorably not more than 80 weight %, still more favorably not more than75 weight %. In addition, this solid component concentration isfavorably in the range of 50 to 82 weight %, more favorably in the rangeof 55 to 82 weight %, still more favorably in the range of 60 to 78weight %. yet still more favorably in the range of 60 to 75 weight %.yet still more favorably in the range of 60 to 73 weight %. yet stillmore favorably in the range of 66 to 73 weight %. In the case where thissolid component concentration is more than 82 weight %, thedeterioration of performance, namely, absorption capacity, and theincrease of the extractable content are seen. In addition, in the casewhere this solid component concentration is less than 50 weight %, thereare disadvantages in that a heavy burden is imposed on the subsequentdrying step.

The above hydropolymer favorably has a form as produced by foamingexpansion and shrinkage during the polymerization. As is illustrated bythe photographs of FIGS. 3 to 7, this is a form which is made as aresult that: the polymerization system foams into diameters on the cm tomm scale with a water vapor pressure due to boiling during thepolymerization to become large in surface area, and further, to therebypromote the volatilization of water, and then the polymerization systemshrinks. In addition, this form exhibits unexpected characteristics ofhaving good releasibility from the polymerization reactor andfacilitating the disintegration of the hydropolymer.

The method for measuring the expansion magnification during thepolymerization is described in the portion hereof under the heading of“DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS”. The expansionmagnification during the polymerization is favorably not less than 2times, more favorably not less than 3 times, still more favorably notless than 5 times, yet still more favorably not less than 10 times, yetstill more favorably not less than 20 times. When expanding, thepolymerization system is subjected to extension force, so thepolymerization proceeds under extension force.

The above hydropolymer is divided into fine pieces, and then dried, andthen pulverized, so that a base polymer (water-absorbent resin beforebeing subjected to surface treatment) can be obtained.

From observation of the resultant base polymer with a microscope, it hasbeen found that even if foaming occurs to the polymerization, most ofparticles are in a noncrystalline form which contains no bubble. Thereason therefor seems to be that the bubble sizes during the foaming arerelatively large.

In the production process according to the present invention, the basepolymer may further be subjected to surface-crosslinking treatment,whereby a water-absorbent resin having a large absorption capacity undera load can be obtained. Usable for this surface-crosslinking treatmentare conventional surface-crosslinking agents and conventionalsurface-crosslinking methods which are usually used for such a purpose.

Important for the production process according to the present inventionis how a hydropolymer having a high solid component concentration of 55to 82 weight % which is formable by polymerization can be disintegrated.In the case where the hydropolymer, which is formed by polymerizing anaqueous solution of water-absorbent resin-forming monomers, has a shapedifficult to dry as it is, such as shapes of thick plates, blocks, andsheets, the hydropolymer is usually disintegrated and then subjected tothe steps such as drying, pulverization, classification, and surfacecrosslinking, thus forming a water-absorbent resin product. In cases ofacrylic acid (salt)-based water-absorbent resins, a hydropolymer havinga solid component concentration of less than 55 weight % can easily bedisintegrated with such as meat-chopper-type disintegrating machines. Inaddition, like a dried hydropolymer, a hydropolymer having a solidcomponent concentration of more than 82 weight % can easily bepulverized with such as conventional impact type pulverizing machines.However, a hydropolymer having a solid component concentration of 55 to82 weight % is difficult to handle because of its properties and state,therefore an attempt to industrially disintegrate such a hydropolymerhas not yet succeeded.

Thus, the present inventors diligently studied about how a hydropolymerhaving a high solid component concentration of 55 to 82 weight % whichis formable by polymerization can be disintegrated. As a result, thepresent inventors have found out that such a hydropolymer can easily bedivided into fine pieces with a specific disintegrating or pulverizingmachine (these are represented by the term “disintegrating machine” inthe present patent application).

Incidentally, the shape of the hydropolymer having a solid componentconcentration of 55 to 82 weight % which is then subjected to thedisintegration is not especially limited, but is favorably the shape ofplates or sheets with a thickness of not larger than 3 cm, morefavorably the shape of plates or sheets with a thickness of not largerthan 2 cm, still more favorably the shape of plates or sheets with athickness of not larger than 1 cm, yet still more favorably the shape ofplates or sheets with a thickness of not larger than 5 cm in a muchwrinkly form as formed by foaming expansion and shrinkage during thepolymerization.

Disintegrating machines having a screen are favorable as apparatuses fordisintegrating the hydropolymer having a solid component concentrationof 55 to 82 weight % in the present invention. More favorable as suchdisintegrating machines are apparatuses corresponding to shearing typecoarsely pulverizing machines or cutting and/or shearing mills which areclassified into pulverizing machines as classified into pulverizingmachines in Table 16 4 in the Chemical Engineering Handbook (6th revisededition, edited by the Chemical Engineering Society of Japan, publishedby Maruzen Co., Ltd. in 1999). Still more favorable are apparatuses tomake disintegration by shearing between a fixed blade and a rotaryblade. The disintegration with these apparatuses provides enablement foreasily disintegrating the hydropolymer having a high solid componentconcentration of 55 to 82 weight % which has so far been difficult todisintegrate.

Specific examples of the shearing type coarsely pulverizing machines orcutting and/or shearing mills include the following:

-   -   SAWS, CIRCULAR SAWS, BAND SAW;    -   VERTICAL PULVERIZER (VERTICAL CUTTING MILL available from Orient        Co., Ltd.);    -   ROTOPLEX available from Hosokawa Mikron Co., Ltd.;    -   TURBO CUTTER available from Turbo Kogyo Co., Ltd.;    -   TURBO GRINDER available from Turbo Kogyo Co., Ltd.;    -   TYRE SHREDDER available from Masuno Seisakusho Co., Ltd.;    -   ROTARY CUTTER MILL available from Yoshida Seisakusho Co., Ltd.;    -   CUTTER MILL available from Tokyo Atomizer Production Co., Ltd.;    -   SHRED CRUSHER available from Tokyo Atomizer Production Co.,        Ltd.;    -   CUTTER MILL available from Masuko Sangyo Co., Ltd.;    -   CRUSHER available from Masuko Sangyo Co., Ltd.;    -   ROTARY CUTTER MILL available from Nara Kikai Seisakusho Co.,        Ltd.;    -   GAINAX CRUSHER available from Horai Co., Ltd.;    -   U-COM available from Horai Co., Ltd.;    -   MESHMILL available from Horai Co., Ltd.;

In the present invention, it has been found out that when thehydropolymer having a solid component concentration in the range of 55to 82 weight % is disintegrated with a disintegrating machine, if acontrivance is carried out in such a manner that the solid componentconcentration is increased by not less than 2 points (for example, theincrease from 70 weight % to 72 weight % in the solid componentconcentration of the hydropolymer by the disintegration of thehydropolymer means an increase of 2 points in the solid componentconcentration) or that a gas, favorably dry air, is passed through thedisintegrating machine or if both contrivances are carried out, then thehydropolymer is disintegrated even with a disintegrating machine ofother than the cutting type wherein such disintegration of thehydropolymer is conventionally difficult.

As the ratio of the increase in the solid component concentration getshigher from 2 points to 3 points, 4 points or as the through-wind amountgets larger, the disintegration becomes easier. However, they should beselected considering economical aspects. When the disintegration iscarried out, the water vapor as emitted from the hydropolymer mightcondense in the apparatus to easily cause the hydropolymer to adhere tothe apparatus and to clog it up, but the through-wind is considered toprevent such phenomena from occurring.

In addition, the disintegration may involve addition of surfactants asdisclosed in JP-A-188726/1999 (Nippon Shokubai Co., Ltd.). However,their necessity becomes less as the solid component concentration of thehydropolymer gets higher.

The weight-average particle diameter of the disintegrated hydropolymeras obtained by the disintegrating means according to the presentinvention is favorably not larger than 100 mm, more favorably not largerthan 10 mm, still more favorably not larger than 3 mm, most favorablynot larger than 1 mm. It is ideal that the disintegration can be madeeven into the particle diameters of end products in a hydropolymerstate.

The residual monomer content of the disintegrated particulatehydropolymer as obtained by the disintegrating means according to thepresent invention is not especially limited, but is not more than 3,000ppm favorably for prevention of the residual monomers from flying aboutin such as the subsequent drying step. According to uses, the residualmonomer content is favorably not more than 1,000 ppm, more favorably notmore than 500 ppm, most favorably not more than 300 ppm. Particularly inthe case where the particulate hydropolymer is intactly used forsanitary articles such as disposable diapers, the residual monomercontent is favorably not more than 1,000 ppm, more favorably not morethan 500 ppm.

It is favorable that the disintegrated hydropolymer (particulatehydropolymer) as obtained by the disintegrating means according to thepresent invention has a solid component concentration in the range of 55to 82 weight %, a residual monomer content of not more than 1,000 ppm,and a weight-average particle diameter of not larger than 3 mm.

Incidentally, the disintegrated hydropolymer (particulate hydropolymer)having a solid component concentration in the range of 55 to 82 weight%, a residual monomer content of not more than 1,000 ppm, and aweight-average particle diameter of not larger than 3 mm, according tothe present invention, does not include those which are obtained byadding water to a hydropolymer which has once got into a dry state(solid component concentration=not less than 83 weight %).

In the production process according to the present invention, thedisintegrated hydropolymer may be dried. The drying method is notespecially limited, but favorable examples thereof include dryingmethods in which the material is sufficiently brought into contact withhot air or a heat transfer surface while being moved, such as agitationdrying methods, fluidized-bed drying methods, and pneumatic dryingmethods.

In the production process according to the present invention, how tosubsequently treat the disintegrated hydropolymer (particulatehydropolymer) can be selected from among the following methods:

-   -   (1) The particulate hydropolymer is intactly made into        manufactured goods, namely, intactly provided to uses such as        sanitary articles or agricultural and horticultural uses. In        order to afford the fluidity to the particles, they may be mixed        with finely-particulate inorganic substances (e.g. bentonite,        zeolite, silicon dioxide).    -   (2) The particulate hydropolymer is mixed with and allowed to        react with a surface-crosslinking agent, and then made into        manufactured goods still in a water-containing state. This        method needs no energy for vaporization of water. In order to        afford the fluidity to the particles, they may be mixed with        finely-particulate inorganic substances (e.g. bentonite,        zeolite, silicon dioxide).    -   (3) The particulate hydropolymer is mixed with and allowed to        react with a surface-crosslinking agent, and dried, and then        made into manufactured goods. In this method, the heating energy        for drying can serve as energy for the surface-crosslinking        reaction, too.    -   (4) The particulate hydropolymer is dried and then intactly made        into manufactured goods.    -   (5) The particulate hydropolymer is dried, and then pulverized,        and then classified, and then made into manufactured goods.    -   (6) The particulate hydropolymer is dried, and then pulverized,        and then classified, and then surface-crosslinked, and then made        into manufactured goods.

Because it has become possible to obtain the particulate hydropolymer bydisintegrating the hydropolymer having a solid component concentrationof 55 to 82 weight % which has so far been difficult to disintegrate,the following have newly become possible.

-   -   1) The above methods (1) to (3) becomes possible.    -   2) It is possible to use the aforementioned drying methods in        which the material is sufficiently brought into contact with hot        air or a heat transfer surface while being moved and which are        therefore good in thermal efficiency (such as agitation drying        methods, fluidized-bed drying methods, and pneumatic drying        methods), but has so far been difficult to use as drying methods        for a hydropolymer having a solid component concentration of        lower than 55 weight % unless a material having a mold-releasing        function such as a surfactant is added thereto.    -   3) Because the disintegration of the polymer can be carried out        in a water-containing state, almost no fine powder is generated,        so a particulate hydropolymer with a low fine-powder content is        obtained.

Another favorable example of the production process, according to thepresent invention, is a process for producing a water-absorbent resinwhich comprises the step of polymerizing an aqueous solution ofwater-absorbent resin-forming monomers including acrylic acid and/or itssalt as major components, wherein:

-   -   (1) the polymerization step produces a hydropolymer having a        solid component concentration in the range of 55 to 82 weight %;    -   and wherein the process further comprises the following steps:    -   (2) a disintegration step for disintegrating the hydropolymer,        which has a solid component concentration in the range of 55 to        82 weight %, into particles of which the weight-average diameter        is not larger than 10 mm; and    -   (3) a drying step for increasing the solid component        concentration in the disintegrated hydropolymer by not less than        3%. The inclusion of these three steps makes it possible to        provide a process by which a water-absorbent resin with        excellent performance is produced at a low cost and further to        provide by reasonable steps the following materials: a base        polymer exhibiting a high absorption capacity without load and        having a low extractable content; and a surface-crosslinked        water-absorbent resin exhibiting a high absorption capacity        under a load.

The present invention further provides a novel water-absorbent resinwhich well functions in uses such as sanitary articles, and is not bulkyand is easily decomposed into linear polymers when being disposed ofafter being used.

As to conventional methods for disposing of water-absorbentresin-containing sanitary articles such as disposable diapers andsanitary napkins after being consumed by users, examples thereofinclude: (1) combustion; (2) landfill; (3) combustion after volumereduction treatment; (4) putting down flush toilets; (5) conversion intocompost; and (6) others. Many studies and investigations are made aboutinfluences of waste water-absorbent resins upon the environment, and itis reported that the influences are on levels which do not matter.However, in the case where used sanitary articles are embedded intosoil, water-absorbent resins absorb water in soil to swell much and aretherefore considered to occupy places and spaces for wastes. Such thingshave not yet mattered, but are desired to also be taken intoconsideration in future, and it is desirable therefor that thewater-absorbent resins become soluble or decompose in the environment.

Thus, many studies are made about biodegradable water-absorbent resins.

For example, they are reported in JP-A-0255896/1999 (Mitsui ChemicalCorporation), JP-A-114803/2001 (Yunichika), JP-A-059820/1996 (NipponShokubai Co., Ltd.), JP-A-196901/1996 (Nippon Shokubai Co., Ltd.),JP-A-089796/1996 (Nippon Shokubai Co., Ltd.), JP-A-124754/1997 (NipponShokubai Co., Ltd.), and JP-A-216914/1997 (Nippon Shokubai Co., Ltd.).However, these are made into manufactured goods of which the prices arehigh because of their expensive raw materials and complicated productionprocesses. In addition, they are inferior to acrylic acid-basedwater-absorbent resins also in respect to performance. Therefore, in thepresent circumstances, they have not yet been put to practical use.

JP-A-104929/2001 (Nippon Asahi Kiko Sale) proposes a process for volumereduction treatment of used disposable diapers, and water-absorbentresins are desired to become less bulky after being used.

Sanitary articles which contain water-absorbent resins and can be putdown flush toilets were also devised (JP-A-210166/1994 and U.S. Pat. No.5,415,643 (Kimberly-Clark)). Because the water-absorbent resins swell indrainpipes, the possibility that the drainpipes might be clogged upbecomes high. Therefore, water-absorbent resins are desired to becomesoluble or decompose when they have been put down flush toilets.

In addition, waste water-absorbent resins involved by the process forproducing water-absorbent resins are thought to be disposed of bycombustion, but there are also attempts to usefully utilize the wastewater-absorbent resins. For example, U.S. Pat. No. 6,143,820 (Dow)discloses an attempt to decompose an acrylic acid-based water-absorbentresin into a linear polymer and to make good use of this linear polymeras a dispersant, a scale inhibitor, or a detergent additive.

The present invention provides a novel water-absorbent resin to solvethese problems.

Specifically, this water-absorbent resin, according to the presentinvention, is obtained by a process including the step of polymerizingan aqueous solution of water-absorbent resin-forming monomers includingacrylic acid and/or its salt as major components, and has the followingproperties:

-   -   (1) 20 (g/g)≦absorption capacity without load (GV)≦60 (g/g);    -   (2) absorption capacity under a load (AAP)≧20 (g/g); and    -   (3) absorption capacity without load (GV)×solubilization residue        ratio (%)≦1,200 ((g/g)%).

The above absorption capacity without load (GV) is favorably in therange of 25 to 55 (g/g), more favorably 25 to 50 (g/g). In the casewhere the absorption capacity without load (GV) is less than 20 (g/g),there are uneconomical disadvantages. In the case where the absorptioncapacity without load (GV) is more than 60 (g/g), there aredisadvantages in that the gel strength is not obtained on a practicaluse level.

The above absorption capacity under a load (AAP) is favorably not lessthan 25 (g/g), more favorably not less than 30 (g/g), still morefavorably not less than 35 (g/g). In the case where the absorptioncapacity under a load (AAP) is less than 20 (g/g), there aredisadvantages in that no favorable performance is exhibited when theresultant water-absorbent resin is used for sanitary articles in a highconcentration.

The value of the above absorption capacity without load(GV)×solubilization residue ratio (%) is favorably not more than 1,000((g/g)%), more favorably not more than 800 ((g/g)%), still morefavorably not more than 600 ((g/g)%). In the case where this value ismore than 1,200 ((g/g)%), there are disadvantages in that the resultantwater-absorbent resin is difficult to decompose and solubilize.

A specific process for producing the above water-absorbent resincomprises the step of polymerizing an aqueous solution ofwater-absorbent resin-forming monomers including acrylic acid and/or itssalt as major components, wherein:

-   -   (1) the aqueous solution has a monomer component concentration        of not less than 50 weight %; and    -   (2) an internal-crosslinking agent is used in a ratio of not        larger than 0.02 mol % to the entirety of the water-absorbent        resin-forming monomers;    -   and wherein the process further comprises the steps of:    -   (3) surface-crosslinking the water-absorbent resin; and    -   (4) adding a chelating agent to the water-absorbent resin in a        ratio of not less than 10 ppm thereto.

More specifically, the monomer component concentration in the aqueoussolution needs not to be less than 50 weight %, and is favorably in therange of 53 to 70 weight %. The ratio of the internal-crosslinking agentto the entirety of the water-absorbent resin-forming monomers needs notto be larger than 0.02 mol %, and is favorably not larger than 0.01 mol%, more favorably not larger than 0.005 mol %. Furthermore, thewater-absorbent resin needs to be surface-crosslinked, and the GV of thesurface-crosslinked resin is favorably not larger than 80%, morefavorably not larger than 70%, still more favorably not larger than 60%,most favorably not larger than 50%, of that of the base polymer.

In the case where a water-absorbent resin is used for sanitary articles,such a water-absorbent resin as decomposes when being used is of nopractical use. Therefore, the addition of the chelating agent to thewater-absorbent resin is needed. The addition of the chelating agent maybe carried out in any step of the process for producing thewater-absorbent resin. In the case where the amount of the chelatingagent as added is less than 10 ppm, the effects are poor. The amount ofthe chelating agent as added is favorably not less than 20 ppm.

(Effects and Advantages of the Invention)

The present invention can provide by reasonable steps the followingmaterials: a base polymer exhibiting a high absorption capacity withoutload and having a low extractable content; and a surface-crosslinkedwater-absorbent resin exhibiting a high absorption capacity under aload.

Because the water-absorbent resin obtained by the present inventionexhibits the above effects, this water-absorbent resin is useful forvarious purposes, for example, as follows: articles contacting humanbodies, such as sanitary articles (e.g. disposable diapers for childrenand adults, sanitary napkins, and adult incontinent products);water-retaining materials for plants and soil; sealing materials forelectric wires and optical cables; and sealing materials for engineeringworks or constructions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following examples of some preferred embodiments in comparison withcomparative examples not according to the invention. However, theinvention is not limited thereto. Incidentally, in the examples, unlessotherwise noted, the unit “part(s)” denotes that by weight.

Measurement of Absorption Capacity without Load (GV))

First, 0.2 g of water-absorbent resin was uniformly placed into anonwoven-fabric-made bag (60 mm×60 mm), and then the bag was immersedinto a 0.9 weight % aqueous sodium chloride solution (physiologicalsaline solution). Thirty minutes later, the bag was drawn up and thendrained at 250×9.81 m/s² (250 G) for 3 minutes with a centrifugalseparator, and the resultant weight W1 (g) of the bag was then measured.On the other hand, the same procedure was carried out without thewater-absorbent resin, and the resultant weight W0 (9) was measured.Thus, the GV (absorption capacity without load) was calculated fromthese weights W1 and W0 in accordance with the following equation:GV (g/g)=[{weight W1(g)−weight W0(g)}/(weight (g) of water-absorbentresin)]−1

In addition, as to the hydropolymer, its GV measurement was carried outin the same way as that for the water-absorbent resin except that: 0.2 gin terms of solid content of the hydropolymer was used; the time of theimmersion in the physiological saline solution was 24 hours; and the GVcalculation involved a correction based on the solid content.

Measurement of Extractable Content and Neutralization Ratio

An amount of 184.3 g of a 0.9 wt % aqueous NaCl solution (physiologicalsaline solution) was weighed out into a plastic container of a capacityof 250 ml with a lid, and then 1.00 g of water-absorbent resin was addedto this aqueous solution, and then they were stirred together for 16hours, whereby extractable components were extracted from the resin. Theresultant dispersion was filtered with filter paper, and 50.0 g of theresultant filtrate was weighed out as a measurement solution.

First of all, the physiological saline solution was titrated alone witha 0.1 N aqueous NaOH solution until pH reached 10, and then with a 0.1 Naqueous HCl solution until pH reached 2.7, thus obtaining blanktitration amounts ([bNaOH] ml and [bHCl] ml).

The same titration procedure as the above was carried out for themeasurement solution, thereby determining its titration amounts ([NaOH]ml and [HCl] ml).

For example, in the case of a water-absorbent resin comprising acrylicacid and its sodium salt, the extractable content and the neutralizationratio of this water-absorbent resin can be calculated from theweight-average molecular weight of the water-absorbent resin and thetitration amounts, as determined by the above procedures, in accordancewith the following calculation formula:Neutralization ratio (mol %)={1−([NaOH]−[bNaOH])/([HCl]−[bHCl])}×100Extractable content (weight%)=0.1×Mw×184.3×100×([HCl]−[bHCl])/1,000/1.0/50.0 whereinMw=72.06×(1−neutralization ratio/100)+94.04×neutralization ratio/100

In addition, as to the hydropolymer, its extractable content measurementwas carried out in the same way as that for the water-absorbent resinexcept that: 1.00 g in terms of solid content of the hydropolymer wasused; the time of the immersion in the physiological saline solution was24 hours; and the extractable content calculation involved a correctionbased on the solid content.

GEX Value

When the GV value and the extractable content of the base polymer aredenoted by y (g/g) and x (weight %) respectively, the GEX value isdefined by the following equation:GEX value=(y−15)/ln(x)

-   -   wherein: In(x) is a natural logarithm of x.

The GEX value is an index for denoting by one parameter an evaluation ofregarding a low extractable content for GV value as good and a highextractable content for GV value as bad in relations between the GVvalue and the extractable content. The larger this GEX value is, thehigher the performance is.

Measurement of Residual Monomer Content

First, 0.5 g of water-absorbent resin was added to 1,000 g of deionizedwater to carry out extraction under stirring for 2 hours. Then, theresultant swollen gelled water-absorbent resin was filtered off withfilter paper, and then the residual monomer content of the resultantfiltrate was analyzed by liquid chromatography. On the other hand, acalibration curve which was obtained by analyzing a monomer standardsolution of an already known concentration in the same way as the abovewas taken as an external standard. Therefrom, the residual monomercontent of the water-absorbent resin was determined in consideration ofthe dilution magnification of the filtrate.

In addition, as to the hydropolymer, its residual monomer contentmeasurement was carried out in the same way as that for thewater-absorbent resin except that: 0.5 g in terms of solid content ofthe hydropolymer was used; the time of the immersion in thephysiological saline solution was 24 hours; and the residual monomercontent calculation involved a correction based on the solid content.

Measurement of Solid Component Concentration in Hydropolymer

A bit of a portion of the hydropolymer as got out of the polymerizationreactor was cut off and then quickly cooled and then quickly dividedinto fine pieces with scissors. Next, 5 g of the hydropolymer as finelydivided in this way was placed into a Petri dish and then dried in adrying oven of 180° C. for 24 hours to calculate the solid componentconcentration in the hydropolymer. As to a particulate hydropolymer, thesolid component concentration therein was calculated by placing 5 g ofsample into a Petri dish and then drying it in a drying oven of 180° C.for 24 hours.

Calculation of Concentration Ratio

Calculated is a ratio (concentration ratio) between a solid componentconcentration in the hydropolymer as formed by polymerization and asolid component concentration in the aqueous monomer solution.Incidentally, the solid components in the aqueous monomer solution aremonomers and other additives, and do not include water or solvents. Forexample, in the case where the solid component concentration in theaqueous monomer solution is 40 weight % and where the solid componentconcentration in the formed hydropolymer is 48 weight %, theconcentration ratio is 48/40=1.20.

Measurement of Absorption Capacity under Load (AAP)

First, 0.9 g of water-absorbent resin is uniformly spread on a stainlessmetal gauze of 400 mesh (mesh opening size: 38 μm) as attached by fusionto the bottom of a plastic supporting cylinder with an inner diameter of60 mm. Next, a piston and a load are mounted in sequence on the abovewater-absorbent resin, wherein the piston has an outer diameter of onlya little smaller than 60 mm and makes no gap with the wall face of thesupporting cylinder, but is not hindered from moving up and down, andwherein the total weight of the piston and the load is adjusted to 565 gso that a load of 20 g/cm² (corresponding to 1.96 kPa) can uniformly beapplied to the water-absorbent resin. Then, the weight (Wa) of theresultant set of measurement apparatus is measured.

A glass filter plate of 90 mm in diameter is mounted inside a Petri dishof 150 mm in diameter, and a 0.9 weight % aqueous NaCl solution is addedup to the same level as the surface of the glass filter plate, on whichfilter paper of 90 mm in diameter is then mounted so that its entiresurface will be wetted, and further, an excess of liquid is removed.

The above set of measurement apparatus is mounted on the above wetfilter paper, thereby allowing the water-absorbent resin to absorb theliquid under load. Then, 1 hour later, the set of measurement apparatusis removed by being lifted to measure its weight (Wb) again.

The absorption capacity under load (AAP) is determined from thefollowing equation:AAP (g/g)=(Wb−Wa)/0.9Measurement of Temperature of Polymerization System

A PC card type data-collecting system, NR-1000 produced by Keyence Co.,Ltd., was used to measure the temperature of a system exhibiting a rapidchange of temperature. A thermocouple was put in the central portion ofthe polymerization system to measure its temperature at a samplingperiod of 0.1 second. The polymerization initiation temperature and thepeak temperature (highest temperature) were read from the resultanttemperature-time chart.

Polymerization Time

Measured is a period of time of from completion of the polymerizationinitiation conditions following the charge of the aqueous monomersolution into a polymerization reactor (from initiation of irradiationof light in the case where the at least one photoinitiator is used; orfrom charge of the aqueous monomer solution and a polymerizationinitiator into a polymerization reactor in the case where nophotoinitiator is used) till attainment to the peak temperature. Namely,the total of Induction period)+(period of time of from polymerizationinitiation till peak temperature) is measured.

Ratio of Temperature Rising during Polymerization

The ratio of temperature rising during the polymerization is a ratio ofΔT (difference between the highest temperature during the polymerizationand the polymerization initiation temperature; ° C.), as observed forthe polymerization system, to theoretical ΔT (° C.) as follows:Ratio of temperature rising during polymerization=(observed ΔT (°C.))/(theoretical ΔT (° C.))whereintheoretical ΔT (° C.)=(monomer mol)×(1.85 kcal/mol)/(solid components(kg) in aqueous monomer solution ×0.5 (kcal/° C./kg)+water (kg) inaqueous monomer solution ×1.0 (kcal/° C./kg))Measurement of Dissolved Oxygen Content of Aqueous Monomer Solution

A measurement apparatus (DO meter UD-1 model produced by Central ScienceCo., Ltd.) was used to measure the dissolved oxygen content of theprepared aqueous monomer solution when its liquid temperature was 50° C.by icing the solution while gently stirring it so as not to minglebubbles thereinto under a nitrogen atmosphere.

Increase of Neutralization Ratio

The difference between the neutralization ratio of the base polymer andthat of the monomers is denoted by points. For example, in the casewhere the neutralization ratio of the base polymer is 65 mol % and wherethat of the monomers is 60 mol %, the increase of the neutralizationratio is 5 points.

Measurement of Expansion Magnification during the Polymerization

There is a case where the polymerization system expands, because waterboils during the polymerization. Scales are made lengthways andwidthways at intervals of 1 cm in the polymerization reactor, and thevolume of the polymerization system is determined by measuring its sizewith the eye when the polymerization system has expanded to the maximumduring the polymerization. Then, the ratio of the determined maximumvolume of the polymerization system to the volume of the aqueous monomersolution is calculated as follows:Expansion magnification (times) during the polymerization=(maximumvolume of polymerization system)/(volume of aqueous monomer solution)Measurement of Particle Diameter Distribution of ParticulateHydropolymer

About 300 g of particulate hydropolymer is placed into a plastic bag,and thereto 1 g of Aerosil R-972 (hydrophobic fine particles of silicondioxide, produced by Nippon Aerosil Co., Ltd.), and they are mixedtogether and well disintegrated by hand. The resultant disintegratedproduct is shaken with a standard screen of 20 cm in inner diameter anda Ro-Tap type screen shaker for 10 minutes. Depending on the watercontent of the particulate hydropolymer, there is a case where theparticulate hydropolymer aggregates so much when being screen-shaken asto make it difficult to precisely measure the particle diameterdistribution. Therefore, the measurement is carried out by adding theAerosil R-972 to the particulate hydropolymer.

Solubilization Test Method

The water-absorbent resin basically has a form of a crosslinked polymeras produced by crosslinking of a water-soluble polymer. Thissolubilization test method is a test method for evaluating how easilythis water-absorbent resin decomposes when exposed to decomposingconditions. Accordingly, according to decomposing conditions to whichthe water-absorbent resin is actually exposed, the tendency does notnecessarily give agreement with the result of this test method.

In this test method, the water-absorbent resin is decomposed by washingsuch as extractable components off with water to obtain a gel comprisinga crosslinked polymer itself, and then irradiating it with ultravioletrays. This evaluation method gives a quantitative index for the ease ofdecomposition of the crosslinked polymer itself. The procedure thereforis as follows.

1. The water-absorbent resin is classified into the particle diameterrange of 300 to 850 μm.

2. Next, 0.500 g of the classified resin is dispersed into 1,000 cc ofion-exchanged water in a PP (polypropylene)-made cylindrical containerand thereby swollen with the ion-exchanged water. This operation iscarried out under stirring with a magnetic stirrer (600 rpm).

3. The stirring is continued at normal temperature for 2 hours.

4. The resultant dispersion is poured onto a circular standard sieve(mesh opening size=300 μm) of 20 cm in diameter, and water is drainedoff by tapping the sieve by hand. Then, the gel as left on the sieve isreturned into the PP-made cylindrical container, and then ion-exchangedwater is added thereto till the entire contents of the container amountsto 1,000 cc, and then the stirring is carried out again for 2 hours.

5. This washing operation is carried out three times, and then thewater-drained gel is placed into a glass Petri dish of 153 mm indiameter.

6. A mercury lamp, H400BL (produced by Toshiba Lightech Co., Ltd.), isfitted to a reflective shade, SN4042A (produced by Toshiba Lightech Co.,Ltd.), and then connected to a mercury lamp stabilizer, H4T1B51(produced by Iwasaki Denki Co., Ltd.), and then lighted. The above Petridish is put on a face located 18 cm below the lower end of the mercurylamp. The irradiation energy in a place 18 cm just under the mercurylamp is 40 mW/cm² as measured with a ultraviolet integrationactinometer, UIT-150 (produced by Ushio Denki Co., Ltd.).

7. The gel on the Petri dish is irradiated by the mercury lamp for 30minutes.

8. After the irradiation, the contents of the Petri dish is returnedinto the PP-made cylindrical container, and then ion-exchanged water isadded thereto till the entire contents of the container amounts to 1,000cc, and then the stirring is carried out for 2 hours. The washingoperation is carried out three times in the same way as of the aboveoperations 4 and 5.

9. The gel as left on the standard sieve (mesh opening size=300 μm) isplaced into a glass Petri dish of 153 mm in diameter and then solidifiedby drying in a drying oven of 180° C. for 5 hours to determine a solidcontent (a).

10. Separately, the above operations 1 to 5 are carried out to obtain agel, and its solid content (b) (before UV irradiation) is determined inthe same way as of the above operation 9.

11. The solubilization residue ratio is calculated from solubilizationresidue ratio=a/b×100 (%).

EXAMPLE 1

A stainless beaker of an inner diameter of 10 cm was fitted with apolystyrene foam-made lid as equipped with a nitrogen-introducing tube,an exhaust tube, and a thermometer. Furthermore, the whole stainlessbeaker was covered with polystyrene foam which is a heat insulator.Then, the beaker was charged with 40.6 g of a 80 weight % aqueousacrylic acid solution in which 0.09 g of polyethylene glycol diacrylate(number-average molecular weight=478) was dissolved. While stirring wasbeing done with a magnetic stirrer, neutralization was carried out byadding a product as obtained by diluting 28.2 g of a 48 weight % aqueoussodium hydroxide solution with 31.0 g of ion-exchanged water. As aresult, the internal temperature reached 90° C. The neutralization ratioof the resultant aqueous monomer solution was 75 mol %. Then, whilenitrogen was introduced, 0.45 g of a 10 weight % aqueous sodiumpersulfate solution was added. Immediately thereafter, polymerizationwas initiated (polymerization initiation temperature=90° C.), and thenthe polymerization system reached the polymerization peak temperature(108° C.) while emitting water vapor. The time as needed from theaddition of the aqueous sodium persulfate solution till thepolymerization peak temperature, that is, the polymerization time, was 2minutes. After the attainment to the polymerization peak temperature,the heat insulating state was still retained for 5 minutes. Theresultant hydropolymer was got out and then divided into fine pieceswith scissors. As a result, 7 minutes were needed from adding thepolymerization initiator till getting out the hydropolymer. The finelydivided hydropolymer was dried with a hot-air oven of 170° C. for 40minutes and then pulverized with a laboratory pulverizer. Next, thepulverized product was classified with screen meshes of the mesh openingsizes of 600 μm and 300 μm, thus obtaining a base polymer (1), most ofwhich had particle diameters of 300 to 600 μm.

The base polymer (1) had a GV of 47 g/g, an extractable content of 10weight %, a residual monomer content of 300 ppm, and a neutralizationratio of 77 mol %. In addition, the finely divided hydropolymer had asolid component concentration of 48 weight %. The concentration ratiowas 1.20.

Next, 100 parts of the base polymer (1) was mixed with asurface-crosslinking agent composition solution comprising 0.05 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 2parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. for 40 minutes, thus obtaining a surface-crosslinkedwater-absorbent resin (1) of which the surface vicinity had beencrosslinked.

The surface-crosslinked water-absorbent resin (1) had a GV of 39 g/g andan AAP of 38 g/g.

The results are shown in Table 1.

In addition, a side view photograph of the hydropolymer as obtained inExample 1 is shown in FIG. 2. The lateral bar below the center of thisphotograph of FIG. 2 is 1 cm long.

COMPARATIVE EXAMPLE 1

A polymerization apparatus, as prepared by fitting the same stainlessbeaker as that used in Example 1 with a polystyrene foam-made lid asequipped with a nitrogen-introducing tube, an exhaust tube, and athermometer, was immersed into a water bath of 20° C. Then, thepolymerization apparatus was charged with the same aqueous monomersolution as that used in Example 1. Then, while nitrogen was introduced,0.45 g of a 10 weight % aqueous sodium persulfate solution and 0.45 g ofa 0.1 weight % aqueous L-ascorbic acid solution were added. Sevenminutes later, polymerization was initiated. The polymerization wascarried out while the polymerization system was cooled with the waterbath of 20° C., so that the polymerization system reached thepolymerization peak temperature of 60° C. Thereafter, heating wascarried out for 30 minutes by raising the temperature of the water bathto 70° C.

Thereafter, the resultant hydropolymer was got out and then divided intofine pieces with scissors. As a result, 62 minutes were needed fromadding the polymerization initiator system till getting out thehydropolymer. The finely divided hydropolymer was dried with a hot-airoven of 170° C. for 40 minutes and then pulverized with a laboratorypulverizer. Next, the pulverized product was classified with screenmeshes of the mesh opening sizes of 600 μm and 300 μm, thus obtaining acomparative base polymer (1), most of which had particle diameters of300 to 600 μm.

The comparative base polymer (1) had a GV of 36 g/g, an extractablecontent of 11 weight %, a residual monomer content of 300 ppm, and aneutralization ratio of 75 mol %. In addition, the finely dividedhydropolymer had a solid component concentration of 42 weight %. Theconcentration ratio was 1.05.

Next, 100 parts of the comparative base polymer (1) was mixed with asurface-crosslinking agent composition solution comprising 0.05 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 2parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. for 40 minutes, thus obtaining a comparativesurface-crosslinked water-absorbent resin (1) of which the surfacevicinity had been crosslinked.

The comparative surface-crosslinked water-absorbent resin (1) had a GVof 28 g/g and an MAP of 26 g/g.

The results are shown in Table 1.

Between Example 1 and Comparative Example 1, such as the composition ofthe monomers and the amount of the initiator are the same, and only thepolymerization initiation temperature (90° C. in Example 1 and 20° C. inComparative Example 1) and the subsequent temperature are different.Example 1 is superior in respect to the performance of the resultantbase polymer and water-absorbent resin. Although the reason therefor isnot clear, the present inventors' inference is as follows.

The polymerization initiation at 20° C. results in the deterioration ofthe GV, because the polymer as formed in the stage of a lowpolymerization conversion has too high a molecular weight, and becausemuch of the crosslinking agent is consumed in the initial stage of thepolymerization (reference: Crosslinker Reactivity and the Structure ofSuperabsorbent Gels, D. J. Arriola et al., J. Appl. Polym. Sci. 63,439-451 (1997)). On the other hand, as to the polymerization initiationat 90° C., the deterioration of the GV is suppressed, because themolecular weight of the polymer as formed in the stage of a lowpolymerization conversion is suppressed.

EXAMPLE 2

An aqueous monomer solution having a monomer concentration of 50 weight% and a neutralization ratio of 65 mol % was prepared by mixing together83.5 parts of acrylic acid, 62.1 parts of a 48.5 weight % aqueous NaOHsolution, 54.3 parts of ion-exchanged water, 0.11 part of polyethyleneglycol diacrylate (number-average degree of polymerization of ethyleneoxide=8) as a crosslinking agent, and 0.01 part of2-hydroxy-2-methyl-1-phenyl-propan-1-one as an initiator. This solutionwas deaerated under a nitrogen atmosphere for 30 minutes and then pouredinto a Teflon-coated stainless container having a bottom of 200×260 mm,wherein the container was put on a hot plate (NEO HOTPLATE HI-1000produced by Inouchi Seieido Co., Ltd.) of 90° C. and nitrogen gas wasbeing introduced into the container. When the temperature of the aqueousmonomer solution rose to 60° C., this solution was irradiated withultraviolet rays by four black light fluorescent lamps CFL6BLB producedby Toshiba Lightech Co., Ltd.) for 10 minutes (quantity of light=900mJ/cm²), thus obtaining a hydropolymer of about 3 mm in thickness. Thepolymerization initiation temperature was 60° C., and the highesttemperature was 110° C. during the polymerization, and thepolymerization time was 80 seconds, and the expansion magnification was5 times. Immediately after the end of the ultraviolet irradiation, thehydropolymer was got out and then cut into fine pieces with scissors andthen dried with hot air of 170° C. for 30 minutes and then pulverizedwith a laboratory pulverizer. Next, the pulverized product wasclassified with screen meshes of the mesh opening sizes of 600 μm and300 μm, thus obtaining a base polymer (2), most of which had particlediameters of 300 to 600 μm.

The base polymer (2) had a GV of 58 g/g, an extractable content of 16weight %, a neutralization ratio of 68 mol %, and a residual monomercontent of 2,200 ppm. In addition, the hydropolymer had a solidcomponent concentration of 60 weight %. The concentration ratio was1.20.

Next, 100 parts of the base polymer (2) was mixed with asurface-crosslinking agent composition solution comprising 0.05 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 2parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. for 40 minutes, thus obtaining a surface-crosslinkedwater-absorbent resin (2) of which the surface vicinity had beencrosslinked.

The surface-crosslinked water-absorbent resin (2) had a GV of 42 g/g andan AAP of 36 g/g.

The results are shown in Table 1.

EXAMPLE 3

A solution (A) was prepared by mixing together 139.5 g of acrylic acid,0.09 g of polyethylene glycol diacrylate (number-average molecularweight=478), and 0.02 g of 2-hydroxy-2-methyl-1-phenyl-propan-1-one. Inaddition, an aqueous NaOH solution (B) was prepared by diluting 95.8 gof a 48.5 weight % aqueous NaOH solution with 61.2 g of ion-exchangedwater and then adding 0.02 g of diethylenetriaminepentaacetic acidpentasodium salt to the resultant dilution. These solutions weredeaerated under a nitrogen atmosphere for 30 minutes. Then, whilestirring was carried out with a magnetic stirrer, the solution (A) wasadded to the solution (B) in an open system all at once to mix themtogether. A deposit was seen in the initial stage of the mixing step,but instantly dissolved to give an aqueous monomer solution (monomerconcentration=55 weight % and neutralization ratio=60 mol %) of whichthe liquid temperature had risen to about 90° C. due to a heat ofneutralization and a heat of dissolution. Furthermore, 0.58 g of a 10weight % aqueous sodium persulfate solution was added to this aqueousmonomer solution, and then the resultant mixture was stirred for severalseconds and then immediately poured into a stainless vat type containerhaving a bottom of 200×260 mm (surface temperature=about 64° C.) in anopen system (thickness of poured solution=about 5 mm), wherein thecontainer was put on a hot plate of 90° C. and a silicone sheet wasattached to the inner surface of the container. The stainless vat typecontainer had the measurements of bottom=200×260 mm, top=560×460 mm,height=140 mm, and was trapezoidal at the central section, and was openat the top. Immediately thereafter, ultraviolet irradiation was carriedout with a black light mercury lamp (peak wavelength=352 nm, model No.H400BL, fitted within a projector MT-4020, wherein both the lamp and theprojector were products of Toshiba Lightech Co., Ltd.) to initiatepolymerization. While the polymerization system was emitting water vaporand expanding in all directions and foaming, the polymerizationproceeded, and then the polymerization system shrank to almost the samesize as the original. The resultant hydropolymer expanded to about 30times at the maximum of the volume of the aqueous monomer solutionaccording to the eye measurement and then shrank. When the hydropolymerexpands, thin portions of the hydropolymer creep up tilt portions of thesides of the container and then, when the hydropolymer shrinks, the thinportions of the hydropolymer return toward their original places, butstop their movements as they are larger than the size of the bottom ofthe container. This expansion and shrinkage ended within about 1 minuteand, when the UV irradiation for 2 minutes had been completed, thehydropolymer was got out. From the record of the change in temperatureof the polymerization system, it was read that the polymerizationinitiation temperature was 88° C. and that the highest temperature was111° C. The resultant hydropolymer was in a much wrinkly form eitherstill in a foamed state or in a collapsed state, although it wasaccording to sizes of bubbles. This hydropolymer was pulverized withVERTICAL PULVERIZER (model No. VM27-S produced by Orient Co., Ltd.,screen mesh opening diameter=8 mm), thus obtaining a flowableparticulate hydropolymer (3).

The hydropolymer (3) had a GV of 33 g/g, an extractable content of 6weight %, and a residual monomer content of 600 ppm. In addition, thehydropolymer had a solid component concentration of 70 weight %. Theconcentration ratio was 1.27.

Subsequently, the hydropolymer (3) was dried with hot air of 170° C. for20 minutes and then pulverized with a roll mill. Next, the pulverizedproduct was classified with screen meshes of the mesh opening sizes of850 μm and 150 μm, thus obtaining a base polymer (3), most of which hadparticle diameters of 150 to 850 μm, and which had a weight-averageparticle diameter of 360 μm.

The base polymer (3) had a GV of 48 g/g, an extractable content of 24weight %, a neutralization ratio of 65 mol %, and a residual monomercontent of 200 ppm. In addition, from observation of particles of thebase polymer (3) with a microscope, it was found that: foaming occurredto the polymerization, but most of particles were in a noncrystallineform which contained no bubble nevertheless. The reason therefor seemsto be that their bubble sizes were relatively large.

Next, 100 parts of the base polymer (3) was mixed with asurface-crosslinking agent composition solution comprising 0.03 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 5parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. in a sealed container for 1 hour, thus obtaining asurface-crosslinked water-absorbent resin (3) of which the surfacevicinity had been crosslinked.

The surface-crosslinked water-absorbent resin (3) had a GV of 34 g/g andan AAP of 35 g/g.

Furthermore, 100 parts of the base polymer (3) was mixed with asurface-crosslinking agent composition solution comprising 0.03 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, 3 parts ofwater, and 0.9 part of isopropyl alcohol, and then the resultant mixturewas heated in a container as heated with an oil bath of 170° C. for 20minutes, thus obtaining a surface-crosslinked water-absorbent resin (3a)of which the surface vicinity had been crosslinked.

The surface-crosslinked water-absorbent resin (3a) had a GV of 34 g/gand an AAP of 35 g/g.

The results are shown in Table 1.

In addition, a side view photograph, a top view photograph, and a bottomview photograph of the hydropolymer as obtained in Example 3 are shownin FIGS. 3, 4, and 5 respectively. The lateral bars below the centers ofthese photographs of FIGS. 3, 4, and 5 are 1 cm long.

EXAMPLE 4

The polymerization was carried out in the same way as of Example 3, andthe evaporated water vapor was led with a fan and collected into acondenser (as cooled with ice water). The amount of an aqueous solutionas collected by carrying out this procedure twice was 60 g, of which 3.4weight % was acrylic acid.

The same procedure as of Example 3 was carried out except that thisrecovered water containing acrylic acid was substituted for most ofion-exchanged water as used in Example 3. The property values of theresultant base polymer (4) are shown in Table 1.

EXAMPLE 5

A base polymer (5) was obtained in the same way as of Example 3 exceptthat 0.09 g of polyethylene glycol diacrylate (number-average molecularweight=478) (crosslinking agent) was replaced with 0.14 g oftrimethylolpropane triacrylate (molecular weight=296). Then, 100 part ofthe resultant base polymer (5) was mixed with a surface-crosslinkingagent composition solution comprising 1 part of propylene glycol, 0.5part of 1,4-butanediol, 3 parts of water, and 1 part of isopropylalcohol, and then the resultant mixture was heated at 210° C. for 40minutes, thus obtaining a surface-crosslinked water-absorbent resin (5).

The results are shown in Table 1.

In addition, a side view photograph of the hydropolymer as obtained inExample 5 is shown in FIG. 6. The lateral bar below the center of thisphotograph of FIG. 6 is 1 cm long.

EXAMPLE 6

An amount of 100 parts of the base polymer (5) as obtained in Example 5was mixed with a surface-crosslinking agent composition solutioncomprising 3 parts of ethylene carbonate, 3 parts of water, and 1 partof isopropyl alcohol, and then the resultant mixture was heated at 210°C. for 40 minutes, thus obtaining a surface-crosslinked water-absorbentresin (6).

The results are shown in Table 1.

TABLE 1 Example Example Example Example Example Example Comparative 1 23 4 5 6 Example 1 Concentration 40 50 55 55 55 55 40 (weight %) inaqueous monomer solution Neutralization 75 65 60 60 60 60 75 ratio (mol%) of monomers Polymerization 90 60 88 87 89 89 20 initiationtemperature (° C.) Polymerization 108 110 111 112 110 110 60 peaktemperature (° C.) ΔT (° C.) 18 50 22 25 21 21 40 Polymerization 120 8039 48 43 43 1,500 time (seconds) Solid component 48 60 70 69 68 68 42concentration (weight %) in hydropolymer Concentration 1.20 1.20 1.271.25 1.24 1.24 1.05 ratio GV g/g of base 47 58 48 50 29 29 36 polymerExtractable 10 16 24 23 4 4 11 content (weight %) of base polymer GEXvalue of 13.9 15.5 10.4 11.2 10.1 10.1 8.8 base polymer Neutralization77 68 65 64 64 64 75 ratio (mol %) of base polymer Residual 300 2,200200 300 270 270 300 monomer content (ppm) GV g/g of 39 42 34 — 25 23 28surface-crosslinked water-absorbent resin AAP (g/g) of 38 36 35 — 26 2426 surface-crosslinked water-absorbent resin

EXAMPLES 7 TO 10

Base polymers of Examples 7 to 10 (base polymers (7) to (10)) wereobtained by carrying out the polymerization and the subsequent operationin the same way as of Example 3 except that the polymerizationinitiation temperature was changed by adjusting the temperature of theaqueous monomer solution and that of the hot plate. The results arecompiled in Table 2.

In addition, a side view photograph of the hydropolymer as obtained inExample 7 is shown in FIG. 7. In addition, a photograph of a gel asformed by cutting off a portion of the hydropolymer resultant fromExample 7 and then swelling it with tap water is shown in FIG. 8. Inthis photograph of FIG. 8, the hydropolymer outside the Petri dish isthe same cut section as that of the hydropolymer which has not yet beenswollen with tap water. The longitudinal bars at the lower left of thesephotographs of FIGS. 7 and 8 are 1 cm long.

TABLE 2 Example Example Example Example Comparative 7 8 9 10 Example 2Concentration 55 55 55 55 70 (weight %) in aqueous monomer solutionNeutralization 60 60 60 60 75 ratio (mol %) of monomers Polymerization88 73 60 44 56 initiation temperature (° C.) Polymerization 111 139 137121 145 peak temperature (° C.) ΔT (° C.) 23 66 73 75 89 Polymerization45 60 86 110 35 time (seconds) Solid component 71 68 66 64 89concentration (weight %) in hydropolymer Concentration 1.29 1.24 1.201.16 1.27 ratio GV (g/g) of base 51 48 45 47 23 polymer Extractable 2530 28 29 50 content (weight %) of base polymer GEX value of 11.2 9.7 9.09.5 2.0 base polymer Neutralization 65 65 65 64 77 ratio (mol %) of basepolymer Increase points 5 5 5 4 2 of neutralization ratio Residual 3002,000 1,600 500 5,900 monomer content (ppm)

In addition, a graph which illustrates relations between polymerizationreaction temperature and time for the base polymers of Examples 3 and 7to 10 is shown in FIG. 1.

In Examples 7 to 10, the polymerization initiation temperature wassimply changed to 88° C., 73° C., 60° C., and 44° C. under conditionswhere the composition of the monomers was the same. As the initiationtemperature gets higher, the performance of the base polymer getsbetter, and the solid component concentration of the hydropolymer alsobecomes higher. In addition, the peak temperature gets lower as theinitiation temperature gets higher. The reason for such phenomena is notclear, but the present inventors' inference is as follows.

In the case where the polymerization initiation temperature is low, ahard hydropolymer is formed no later than the attainment to boilingafter the initiation, so the boiling temperature becomes high, in otherwords, the peak temperature also becomes high, and a large plosive ismade. In the case where the polymerization initiation temperature ishigh, no hard hydropolymer is formed no later than the attainment toboiling after the initiation, and an aqueous monomer solution having ahigh viscosity foams so much with boiling as to enlarge in surface areato emit water vapor well, and the peak temperature is depressed, and thesolid component concentration also becomes higher. In addition, almostno plosive is made.

COMPARATIVE EXAMPLE 2

A potassium acrylate solution having a mixed monomer concentration of 70weight % (neutralization ratio=75 mol %) was prepared by adding 72.1 gof acrylic acid to 22.2 g of ion-exchanged water, and then addingthereto 49.5 g of potassium hydroxide of the purity of 85% as aneutralizing agent and 0.01 g of N,N′-methylenebisacrylamide as adivinyl compound in order.

The aqueous solution as prepared in the above way was kept at 70° C. andthen mixed with 2.9 g of a 18 weight % aqueous ammonium persulfatesolution (0.5 weight % of the total weight of potassium acrylate, freeacrylic acid, and N,N′-methylenebisacrylamide (total weight of themonomer components)) and 1.7 g of a 30.6 weight % aqueous sodiumhydrogensulfite solution (0.5 weight %). The resultant mixture solutionwas placed into a stainless beaker (capacity=2 liters, φ=135 mm) and, asa result, became a layer of about 10 mm in thickness. Then, about 7seconds later, a polymerization reaction was initiated and thencompleted in about 1 minute, while the highest temperature was 145° C.

The resultant hydropolymer was solid and easy to pulverize. Thus, acomparative base polymer (2) was obtained, which had a solid componentconcentration of 89 weight %, a GV of 23 g/g, an extractable content of50 weight %, a neutralization ratio of 77 mol %, and a residual monomercontent of 5,900 ppm.

The results are compiled in Table 2.

This Comparative Example is a trace of Example 1 as set forth inJP-A-071907/1983 (Arakawa Kagaku). It has been found that if the solidcomponent concentration increases to 89 weight %, the extractablecontent increases greatly.

EXAMPLE 11

A solution (A) was prepared by mixing together 139.5 g of acrylic acid,0.09 g of polyethylene glycol diacrylate (number-average molecularweight=478), and 0.02 g of 2-hydroxy-2-methyl-1-phenyl-propan-1-one. Inaddition, an aqueous NaOH solution (B) was prepared by diluting 95.8 gof a 48.5 weight % aqueous NaOH solution with 64.1 g of ion-exchangedwater and then adding 0.02 g of diethylenetriaminepentaacetic acidpentasodium salt to the resultant dilution. These solutions weredeaerated under a nitrogen atmosphere for 30 minutes. Then, whilestirring was carried out with a magnetic stirrer, the solution (A) wasadded to the solution (B) in an open system all at once to mix themtogether. A deposit was seen in the initial stage of the mixing step,but instantly dissolved to give an aqueous monomer solution (monomerconcentration=55 weight % and neutralization ratio=60 mol %) of whichthe liquid temperature had risen to about 85° C. due to a heat ofneutralization and a heat of dissolution. Furthermore, 0.58 g of a 10weight % aqueous sodium persulfate solution was added to this aqueousmonomer solution, and then the resultant mixture was stirred for severalseconds and then immediately poured into a stainless vat type containerhaving a bottom of 200×260 mm (surface temperature=about 64° C.) in anopen system (thickness of poured solution=about 5 mm), wherein thecontainer was put on a hot plate of 90° C. and a silicone sheet wasattached to the inner surface of the container. The stainless vat typecontainer had the measurements of bottom=200×260 mm, top=560×460 mm,height=140 mm, and was trapezoidal at the central section, and was openat the top. Immediately thereafter, UV irradiation was carried out witha black light mercury lamp (peak wavelength=352 nm, model No. H400BL,produced by Toshiba Lightech Co., Ltd.) to initiate polymerization.While the polymerization system was emitting water vapor and expandingin all directions and foaming, the polymerization proceeded, and thenthe polymerization system shrank to almost the same size as theoriginal. The resultant hydropolymer expanded to about 30 times at themaximum of the volume of the aqueous monomer solution according to theeye measurement and then shrank. When the hydropolymer expands, thinportions of the hydropolymer creep up tilt portions of the sides of thecontainer and then, when the hydropolymer shrinks, the thin portions ofthe hydropolymer return toward their original places, but stop theirmovements as they are larger than the size of the bottom of thecontainer. This expansion and shrinkage ended within about 1 minute and,when the UV irradiation for 2 minutes had been completed, thehydropolymer was got out. Incidentally, from a temperature measurementchart, it was read that the polymerization initiation temperature was82° C. and that the highest temperature was 113° C. The resultanthydropolymer (11) was in a much wrinkly form and had a solid componentconcentration of 70 weight %. Accordingly, the concentration ratio was1.27. This hydropolymer (11) was pulverized with VERTICAL PULVERIZER(model No. VM27-S, screen mesh opening diameter=3 mm, produced by OrientCo., Ltd.), thus obtaining a flowable particulate hydropolymer (11).

The particulate hydropolymer (11) had a weight-average particle diameterof 1 mm, a GV of 33 g/g, an extractable content of 6 weight %, aresidual monomer content of 600 ppm, and a solid component concentrationof 71 weight %.

Subsequently, the particulate hydropolymer (11) was dried with hot airin a drying oven of 170° C. for 20 minutes and then pulverized with aroll mill. Next, the pulverized product was classified with screenmeshes of the mesh opening sizes of 850 μm and 150 μm, thus obtaining abase polymer (11), most of which had particle diameters of 150 to 850μm, and which had a weight-average particle diameter of 360 μm.

The base polymer (11) had a GV of 48 g/g, an extractable content of 24weight %, a neutralization ratio of 65 mol %, and a residual monomercontent of 200 ppm. In addition, from observation of particles of thebase polymer (11) with a microscope, it was found that: foaming occurredto the polymerization, but most of particles were in a noncrystallineform which contained no bubble nevertheless. The reason therefor seemsto be that their bubble sizes were relatively large.

Next, 100 parts of the base polymer (11) was mixed with asurface-crosslinking agent composition solution comprising 0.03 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 5parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. in a sealed container for 1 hour, thus obtaining asurface-crosslinked water-absorbent resin (11) of which the surfacevicinity had been crosslinked.

The surface-crosslinked water-absorbent resin (11) had a GV of 34 gigand an AAP of 32 g/g.

Furthermore, 100 parts of the base polymer (11) was mixed with asurface-crosslinking agent composition solution comprising 0.03 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, 3 parts ofwater, and 0.9 part of isopropyl alcohol, and then the resultant mixturewas heated in a container as heated with an oil bath of 170° C. for 20minutes, thus obtaining a surface-crosslinked water-absorbent resin(11a) of which the surface vicinity had been crosslinked.

The surface-crosslinked water-absorbent resin (11a) had a GV of 34 g/gand an AAP of 35 g/g.

EXAMPLE 12

A solution (A) was prepared by mixing together 308.2 g of acrylic acid,0.20 g of polyethylene glycol diacrylate number-average molecularweight=478), and 0.04 g of 2-hydroxy-2-methyl-1-phenyl-propan-1-one. Inaddition, an aqueous NaOH solution (B) was prepared by diluting 194.1 gof a 48.5 weight % aqueous NaOH solution with 97.0 g of ion-exchangedwater. Then, while stirring was carried out with a magnetic stirrer, thesolution (A) was added to the solution (B) in an open system all at onceto mix them together. A deposit was seen in the initial stage of themixing step, but instantly dissolved to give an aqueous monomer solution(monomer concentration=60 weight %, neutralization ratio=55 mol %,temperature=102° C.) (this aqueous monomer solution preparationoperation was carried out in another batch in the same way, and theamount of dissolved oxygen was measured, so that it was 0.7 ppm).Furthermore, 1.3 g of a 10 weight % aqueous sodium persulfate solutionwas added to this aqueous monomer solution, and then the resultantmixture was stirred for several seconds and then immediately poured intoa stainless vat type container in an open system, wherein the containerwas put on a hot plate of 90° C. and a silicone sheet was attached tothe inner surface of the container. The stainless vat type container hadthe measurements of bottom=200×260 mm, top=560×460 mm, height=140 mm,and was trapezoidal at the central section, and was open at the top.Immediately thereafter, UV irradiation was carried out with a blacklight mercury lamp (peak wavelength=352 nm, model No. H400BL, producedby Toshiba Lightech Co., Ltd.) to initiate polymerization. While thepolymerization system was emitting water vapor and expanding in alldirections and foaming, the polymerization proceeded (expansionmagnification=40 times). When the UV irradiation for 2 minutes had beencompleted, the resultant hydropolymer was got out. This hydropolymer hada solid component concentration of 77 weight %. This hydropolymer waspulverized with a screen of 1 mm in mesh opening diameter in VERTICALPULVERIZER (model No. VM27-S, screen mesh opening diameter=3 mm,produced by Orient Co., Ltd.), thus obtaining a flowable particulatehydropolymer. Then, this particulate hydropolymer was classified withscreen meshes of the mesh opening sizes of 850 μm and 150 μm, thusobtaining a particulate hydropolymer (12) having a weight-averageparticle diameter of 500 μm.

The particulate hydropolymer (12) has a GV of 22 g/g, an extractablecontent of 2 weight %, a residual monomer content of 600 ppm, and asolid component concentration of 79 weight %.

EXAMPLE 13

A solution (A) was prepared by mixing together 279.0 g of acrylic acid,0.09 g of polyethylene glycol diacrylate (number-average molecularweight=478), and 0.03 g of 2-hydroxy-2-methyl-1-phenyl-propan-1-one. Inaddition, an aqueous NaOH solution (B) was prepared by diluting 191.54 gof a 48.5 weight % aqueous NaOH solution with 128.2 g of ion-exchangedwater. Then, while stirring was carried out with a magnetic stirrer, thesolution (A) was added to the solution (B) in an open system all at onceto mix them together. A deposit was seen in the initial stage of themixing step, but instantly dissolved to give an aqueous monomer solution(monomer concentration=55 weight %, neutralization ratio=60 mol %,temperature=92° C.) (this aqueous monomer solution preparation operationwas carried out in another batch in the same way, and the amount ofdissolved oxygen was measured, so that it was 1.4 ppm). Furthermore, 1.3g of a 10 weight % aqueous sodium persulfate solution was added to thisaqueous monomer solution, and then the resultant mixture was stirred forseveral seconds and then immediately poured into a stainless vat typecontainer in an open system, wherein the container was put on a hotplate of 90° C. and a silicone sheet was attached to the inner surfaceof the container. The stainless vat type container had the measurementsof bottom=200×260 mm, top=560×460 mm, height=140 mm, and was trapezoidalat the central section, and was open at the top. Immediately thereafter,UV irradiation was carried out with a black light mercury lamp (peakwavelength=352 nm, model No. H400BL, produced by Toshiba Lightech Co.,Ltd.) to initiate polymerization. While the polymerization system wasemitting water vapor and expanding in all directions and foaming, thepolymerization proceeded (expansion magnification=35 times). When the UVirradiation for 2 minutes had been completed, the resultant hydropolymerwas got out (solid component concentration=69 weight %). Thishydropolymer was pulverized with VERTICAL PULVERIZER (model No. VM27-S,produced by Orient Co., Ltd.) wherein the hydropolymer was firstpulverized with a screen of 3 mm in mesh opening diameter and thenfurther pulverized with a screen of 1 mm in mesh opening diameter, thusobtaining a flowable particulate hydropolymer. Then, this particulatehydropolymer was classified with screen meshes of the mesh opening sizesof 850 μm and 150 μm, thus obtaining a particulate hydropolymer (13)having a weight-average particle diameter of 610 μm (solid componentconcentration=70 weight %).

The particulate hydropolymer (13) has a GV of 34 g/g, an extractablecontent of 10 weight %, and a residual monomer content of 700 ppm.

Next, 100 parts of the particulate hydropolymer (13) was mixed with asurface-crosslinking agent composition solution comprising 0.02 part ofethylene glycol diglycidyl ether and 0.2 part of propylene glycol, andthen the resultant mixture was heated in a drying oven of 80° C. in asealed container for 1 hour. The particles once aggregated togetherafter this heating, but could easily be disintegrated into a particulateform by cooling to room temperature. Thus obtained was asurface-crosslinked water-absorbent resin (13) of which the surfacevicinity had been crosslinked.

The surface-crosslinked water-absorbent resin (13) has a GV of 25 g/g,an AAP of 23 g/g, a residual monomer content of 200 ppm, and a solidcomponent concentration of 74 weight %.

EXAMPLE 14

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw, and thenplaced into TURBO CUTTER (C-300, produced by Turbo Kogyo Co., Ltd.)having a screen mesh opening diameter of 3 mm, and then disintegrated,thus obtaining a particulate hydropolymer (14). About 50 kg of thehydropolymer (11) could be treated in 30 minutes.

The resultant particulate hydropolymer (14) had a solid componentconcentration of 71 weight % and a weight-average particle diameter of1.3 mm. Its particle diameter distribution as determined with a Ro-Taptype screen shaker was as follows.

Particulate hydropolymer having particle diameters of not  9 weight %smaller than 2 mm: Particulate hydropolymer having particle diameters inthe 38 weight % range of 1.4 to 2 mm, but not including 2 mm:Particulate hydropolymer having particle diameters in the 31 weight %range of 1 to 1.4 mm, but not including 1.4 mm: Particulate hydropolymerhaving particle diameters in the 19 weight % range of 0.5 to 1 mm, butnot including 1 mm: Particulate hydropolymer having particle diametersof  3 weight % smaller than 0.5 mm:

EXAMPLE 15

The particulate hydropolymer (14), as obtained by disintegration withthe TURBO CUTTER in Example 14, was placed into TURBO GRINDER (TG-300,produced by Turbo Kogyo Co., Ltd.) and then further disintegrated, thusobtaining a particulate hydropolymer (15).

Shown in Table 4 are: the screen mesh opening diameter of the TURBOGRINDER; and the weight-average particle diameter and the solidcomponent concentration of the disintegrated particulate hydropolymer(15).

EXAMPLE 16

The particulate hydropolymers (14) and (15) were dried with afluidized-bed drier (Pulvis GB22, produced by Yamato Kagaku Co., Ltd.)under conditions of internal temperature=180° C., material=100 g,hot-air flow rate=0.35 m³/minute.

Shown in Table 3 are the following measured values: the materialtemperature during the drying and the solid component concentrationafter the drying of the particulate hydropolymers (14) and (15); and theGV, the extractable content, and the residual monomer content of thedried particulate hydropolymers (14) and (15) as obtained by pulverizingthe dried products and then classifying the pulverized products into theparticle diameter range of 300 to 600 μm.

TABLE 3 Dried particulate Dried particulate hydropolymer (14)hydropolymer (15) Temperature (° C.) of 105 90 materials at drying timeof 1 minute Temperature (° C.) of 140 127 materials at drying time of 2minutes Temperature (° C.) of 159 145 materials at drying time of 3minutes Temperature (° C.) of 171 161 materials at drying time of 5minutes Temperature (° C.) of 179 173 materials at drying time of 10minutes Solid component 95 96 concentration (weight %) after drying for10 minutes GV (g/g) 44 42 Extractable content 18 16 (weight %) Residualmonomer content 250 300 (ppm)

EXAMPLE 17

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw, and thencontinuously supplied into ROTOPLEX (28/40Ro, produced by HosokawaMikron Co., Ltd.) having a screen mesh opening diameter of 2 mm. Thetreatment rate was about 70 kg/hour.

The particulate hydropolymer (17), as obtained by collection with acyclone after the disintegration, had a solid component concentration of72 weight % and a weight-average particle diameter of about 1 mm. Itsparticle diameter distribution as determined with a Ro-Tap type screenshaker was as follows.

Particulate hydropolymer having particle diameters of not  1 weight %smaller than 2 mm: Particulate hydropolymer having particle diameters inthe 29 weight % range of 1.2 to 2 mm, but not including 2 mm:Particulate hydropolymer having particle diameters in the 35 weight %range of 0.85 to 1.2 mm, but not including 1.2 mm: Particulatehydropolymer having particle diameters in the 26 weight % range of 0.3to 0.85 mm, but not including 0.85 mm: Particulate hydropolymer havingparticle diameters of  9 weight % smaller than 0.3 mm:

EXAMPLE 18

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw, and thencontinuously supplied into ROTOPLEX (28/40Ro, produced by HosokawaMikron Co., Ltd.) having a screen mesh opening diameter of 5 mm. Thetreatment rate was about 100 kg/hour.

The particulate hydropolymer (18), as obtained by collection with acyclone after the disintegration, had a solid component concentration of71 weight % and a weight-average particle diameter of 2 mm. Its particlediameter distribution as determined with a Ro-Tap type screen shaker wasas follows.

Particulate hydropolymer having particle diameters of not  6 weight %smaller than 2.8 mm: Particulate hydropolymer having particle diametersin the 58 weight % range of 1.7 to 2.8 mm, but not including 2.8 mm:Particulate hydropolymer having particle diameters in the 29 weight %range of 0.85 to 1.7 mm, but not including 1.7 mm: Particulatehydropolymer having particle diameters in the  6 weight % range of 0.15to 0.85 mm, but not including 0.85 mm: Particulate hydropolymer havingparticle diameters of  1 weight % smaller than 0.15 mm:

Next, this disintegrated particulate hydropolymer (18) was placed intoDRY MEISTER (apparatus to simultaneously carry out pulverization anddrying by hot air and a dispersing rotor, produced by Hosokawa MikronCo., Ltd.) wherein the hot-air temperature was 270° C. and thedispersing rotor was rotated at 3,000 rpm.

The particle diameter distribution of the particulate hydropolymer (18a)(dried and then collected with a cyclone), as determined with a Ro-Taptype screen shaker, was as follows.

Particulate hydropolymer having particle diameters of not  1 weight %smaller than 2.8 mm: Particulate hydropolymer having particle diametersin the  5 weight % range of 1.7 to 2.8 mm, but not including 2.8 mm:Particulate hydropolymer having particle diameters in the 24 weight %range of 0.85 to 1.7 mm, but not including 1.7 mm: Particulatehydropolymer having particle diameters in the 59 weight % range of 0.15to 0.85 mm, but not including 0.85 mm: Particulate hydropolymer havingparticle diameters of 11 weight % smaller than 0.15 mm:

The particulate hydropolymer (18a) had a solid component concentrationof 94 weight % and a weight-average particle diameter of 0.4 mm. Noadhered matter was seen inside the DRY MEISTER after its operation.

This dried and pulverized particulate hydropolymer (18a) had a GV of 40g/g, an extractable content of 15 weight %, and a residual monomercontent of 400 ppm.

EXAMPLE 19

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw, and thenplaced into a laboratory pulverizer ADS-model produced by Miyako BussanCo., Ltd., screen mesh opening diameter=1 mm, hammer crusher typepulverizer), but the hydropolymer instantly jammed and was therefore notdischarged. Thus, a dust collector (Model 406 for both dry and wetbusiness uses, produced by Makita Co., Ltd.) was connected to thedischarging outlet in order to make an air stream in the pulverizer. Asa result, the hydropolymer became smoothly discharged. The dischargedmatter had a solid component concentration of 78 weight % and aweight-average particle diameter of 650 μm, and the ratio of itsportions having particle diameters of not larger than 150 μm was 7weight %.

It is considered that the air streaming during the pulverization couldreduce the adhesion of the materials by quickly carrying away water asemitted by heat generation during the pulverization, so that it becamepossible to do the pulverization.

EXAMPLE 20

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw, and thenplaced into CUTTER MILL (UG03-280LFT, screen mesh opening diameter=8 mm,produced by Horai Co., Ltd.). Incidentally, a blower (DF type fan, DF-3,blow speed=15 m³/min, produced by Horai Co., Ltd.) and a cyclone wereconnected to this CUTTER MILL, and the disintegrated product wasobtained from the lower part of the cyclone. The particulatehydropolymer (20) as obtained by the disintegration had a solidcomponent concentration of 72 weight % and a weight-average particlediameter of 3 mm. Its particle diameter distribution as determined witha Ro-Tap type screen shaker was as follows.

Particulate hydropolymer having particle diameters of not 17 weight %smaller than 4 mm: Particulate hydropolymer having particle diameters inthe 69 weight % range of 2 to 4 mm, but not including 4 mm: Particulatehydropolymer having particle diameters in the 13 weight % range of 1 to2 mm, but not including 2 mm: Particulate hydropolymer having particlediameters of  1 weight % smaller than 1 mm:

Next, this particulate hydropolymer (20) was placed into MESHMILL(HA8-2542, screen mesh opening diameter=2 mm, produced by Horai Co.,Ltd.) and then further disintegrated. The particulate hydropolymer (20a)as collected with a cyclone had a solid component concentration of 75weight % and a weight-average particle diameter of 0.6 mm. The treatmentrate was 130 kg/hour. The particle diameter distribution of thisparticulate hydropolymer (20a), as determined with a Ro-Tap type screenshaker, was as follows.

Particulate hydropolymer having particle diameters of not  0 weight %smaller than 1.4 mm: Particulate hydropolymer having particle diametersin the  8 weight % range of 1 to 1.4 mm, but not including 1.4 mm:Particulate hydropolymer having particle diameters in the 17 weight %range of 0.85 to 1 mm, but not including 1 mm: Particulate hydropolymerhaving particle diameters in the 72 weight % range of 0.15 to 0.85 mm,but not including 0.85 mm: Particulate hydropolymer having particlediameters of  3 weight % smaller than 0.15 mm: No adhered matter wasseen inside the MESHMILL after its operation.

No adhered matter was seen inside the MESHMILL after its operation

EXAMPLE 21

An aqueous monomer solution having a monomer concentration of 50 weight%, a neutralization ratio of 65 mol %, and a temperature of 86° C. wasprepared by mixing together 83.5 parts of acrylic acid, 62.1 parts of a48.5 weight % aqueous NaOH solution, 54.3 parts of ion-exchanged water,0.11 part of polyethylene glycol diacrylate (number-average degree ofpolymerization of ethylene oxide=8) as a crosslinking agent, and 0.01part of 2-hydroxy-2-methyl-1-phenyl-propan-1-one as an initiator (thisaqueous monomer solution preparation operation was carried out inanother batch in the same way, and the amount of dissolved oxygen wasmeasured, so that it was 3.0 ppm). This aqueous monomer solution waspoured into a Teflon-coated stainless container having a bottom of200×260 mm, wherein the container was put on a hot plate of 90° C. andnitrogen gas was being introduced into the container. When thetemperature of the aqueous monomer solution became 80° C., this solutionwas irradiated with ultraviolet rays by a black light fluorescent lampfor 10 minutes (quantity of light=780 mJ/cm²), thus obtaining ahydropolymer (21) of about 3 mm in thickness. This hydropolymer had asolid component concentration of 60 weight %. Accordingly, theconcentration ratio was 1.20. The highest temperature was 110° C. duringthe polymerization. The hydropolymer (21) was cut into fine pieces withVERTICAL PULVERIZER (model No. VM27-S, screen mesh opening diameter=3mm, produced by Orient Co., Ltd.), thus obtaining a particulatehydropolymer (21) having a weight-average particle diameter of 1.6 mmand a solid component concentration of 61 weight %. This particulatehydropolymer (21) was dried with hot air of 170° C. for 30 minutes andthen pulverized with a laboratory pulverizer. Next, the pulverizedproduct was classified with screen meshes of the mesh opening sizes of600 μm and 300 μm, thus obtaining a base polymer (21), most of which hadparticle diameters of 300 to 600 μm.

The base polymer (21) had a GV of 58 g/g, an extractable content of 16weight %, a neutralization ratio of 68 mol %, and a residual monomercontent of 2,200 ppm.

Next, 100 parts of the base polymer (21) was mixed with asurface-crosslinking agent composition solution comprising 0.05 part ofethylene glycol diglycidyl ether, 1 part of propylene glycol, and 2parts of water, and then the resultant mixture was heated in a dryingoven of 80° C. for 40 minutes, thus obtaining a surface-crosslinkedwater-absorbent resin (21) of which the surface vicinity had beencrosslinked.

The surface-crosslinked water-absorbent resin (21) had a GV of 42 g/gand an AAP of 36 g/g.

Table 4 is a compilation about the disintegration of the hydropolymersof Examples 11 to 15 and 17 to 21.

TABLE 4 Screen mesh Solid Solid opening component Weight-averagecomponent diameter (mm) concentration particle concentration of (weight%) in diameter (mm) (weight %) in Disintegrating disintegratingparticulate of particulate hydropolymer machine name machinehydropolymer hydropolymer Example 11 70 VERTICAL 3 71 1 PULVERIZERExample 12 77 VERTICAL 1 79 0.5 PULVERIZER Example 13 69 VERTICAL 3.1 700.61 PULVERIZER Example 14 70 TURBO 3 71 1.3 CUTTER Example 15 71 TURBO1 72 0.82 GRINDER Example 15 71 TURBO 0.7 73 0.58 GRINDER Example 17 70ROTOPLEX 2 72 1 Example 18 70 ROTOPLEX 5 71 2 Example 18 71 DRY MEISTER— 94 0.4 Example 19 70 FDS 1 78 0.65 Example 20 70 CUTTER MILL 8 72 3Example 20 72 MESHMILL 2 75 0.6 Example 21 60 VERTICAL 3 61 1.6PULVERIZER

COMPARATIVE EXAMPLE 3

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw and thenplaced into a meat chopper having dies of the aperture of 8 mm (producedby Hiraga Seisakusho Co., Ltd.). As a result, the burden was so heavy asto immediately stop the operation, therefore the disintegration couldnot be carried out.

COMPARATIVE EXAMPLE 4

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw and thenplaced into a sample mill (KIIW-1 model, produced by Fuji PaudalIndustry Co., Ltd.) having a screen mesh opening diameter of 3 mm. As aresult, the burden was so heavy as to immediately stop the operation,therefore the disintegration could not be carried out.

COMPARATIVE EXAMPLE 5

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was then placed into a kneader (capacity=2.5 liters,produced by Koike Tekko Co., Ltd.) having sigma type blades. As aresult, the hydropolymer was kneaded, but could not be disintegrated.

COMPARATIVE EXAMPLE 6

A sheet of the hydropolymer (11) in a much wrinkly form, as obtained inExample 11, was cut into roughly four with a circular saw and thenplaced into an apparatus of FIG. 4 as drawn in JP-A-188727/1999. As aresult, the hydropolymer twined around the shaft, and therefore couldnot be disintegrated.

COMPARATIVE EXAMPLE 7

A reaction solution was prepared by dissolving 9.25 g of polyethyleneglycol diacrylate (number-average degree of polymerization of ethyleneoxide=8) into 5,500 g of an aqueous solution of sodium acrylate having aneutralization ratio of 55 mol % (monomer concentration=30 weight %).Next, this solution was deaerated under a nitrogen gas atmosphere for 30minutes, and then the resultant solution was supplied into a reactor asprepared by lidding a jacketed stainless-steel-made twin-arm kneader of10 liters in capacity having two sigma type blades. While maintainingthe reaction solution at 30° C., the internal air of the system wasreplaced with nitrogen gas. Next, while the reaction solution wasstirred, 1.91 g of 2,2′-azobis(2-amidinopropane) dihydrochloride, 0.96 gof sodium per sulfate and 0.10 g of L-ascorbic acid were added, with theresult that a polymerization reaction started about 1 minute after.Then, this polymerization reaction was carried out in the range of 30 to80° C. and, 60 minutes after the initiation of the polymerizationreaction, the resultant hydropolymer was got out. The resultanthydropolymer had a finely divided diameter of about 5 mm. This finelydivided hydropolymer was spread onto a 50-mesh metal gauze and thendried at 150° C. with hot air for 90 minutes. Then, the resultant driedproduct was pulverized with a vibration mill and then classified with a20-mesh metal gauze, thus obtaining a comparative base polymer (7) ofthe irregular pulverized shape having a weight-average particle diameterof 300 μm. Next, 100 parts of the resultant comparative base polymer (7)was mixed with a surface-crosslinking agent composition solutioncomprising 0.005 part of diethylenetriaminepentaacetic acid pentasodiumsalt, 1 part of propylene glycol, 0.05 part of ethylene glycoldiglycidyl ether, 3 parts of water, and 1 part of isopropyl alcohol. Theresultant mixture was heated at 210° C. for 45 minutes, thus obtaining acomparative surface-crosslinked water-absorbent resin (7).

COMPARATIVE EXAMPLE 8

An aqueous monomer solution was prepared by mixing 67.0 parts of a 37weight % aqueous sodium acrylate solution, 10.2 parts of acrylic acid,0.155 part of polyethylene glycol diacrylate (number-average degree ofpolymerization of ethylene oxide=8), and 22.0 parts of water together.Nitrogen was blown into the above aqueous monomer solution in a vat,thus reducing the concentration of dissolved oxygen in the aqueousmonomer solution to not higher than 0.1 ppm. Then, the temperature ofthe above aqueous monomer solution was adjusted to 18° C. under nitrogenatmosphere. Next, thereto 0.16 part of a 5 weight % aqueous sodiumpersulfate solution, 0.16 part of a 5 weight % aqueous2,2′azobis(2-amidinopropane) dihydrochloride solution, 0.15 part of a0.5 weight % aqueous L-ascorbic acid solution, and 0.17 part of a 0.35weight % aqueous hydrogen peroxide solution were dropwise added insequence under stirred conditions. After the dropwise addition ofhydrogen peroxide, a polymerization reaction immediately started and, 10minutes later, the temperature of the aqueous monomer solution reachedthe peak temperature of 85° C. Then, the vat was immersed into a hotwater bath of 80° C. and aged for 15 minutes. The resultant transparenthydropolymer was crushed with a meat chopper. The resultant finelydivided hydropolymer was spread onto a 50-mesh metal gauze and thendried at 160° C. with hot air for 65 minutes. Then, the resultant driedproduct was pulverized with a pulverizing machine and then classifiedinto what passed through a screen of 850 μm but remained on a screen of106 μm, thus obtaining a comparative base polymer (8) of the irregularpulverized shape having a weight-average particle diameter of 320 μm.Next, 100 parts of the resultant comparative base polymer (8) was mixedwith a surface-crosslinking agent composition solution comprising 1 partof propylene glycol, 0. 5 part of 1,4-butanediol, 3 parts of water, and1 part of isopropyl alcohol. The resultant mixture was heated at 210° C.for 40 minutes, thus obtaining a comparative surface-crosslinkedwater-absorbent resin (8).

In addition, shown in Table 5 are the results of comparison of GV, AAP,solubilization residue ratio, GV×solubilization residue ratio among thesurface-crosslinked water-absorbent resins (2), (3), (5), (6) ofExamples 2, 3, 5, 6, comparative surface-crosslinked water-absorbentresins (1), (7), (8) of Comparative Examples 1, 7, 8, comparative basepolymers (1), (7), (8) of Comparative Examples 1, 7, 8, andwater-absorbent resins which are products of other makers.

TABLE 5 Synthetic Solubilization example or GV AAP residue ratio GV ×solubilization maker name (g/g) (g/g) (%) residue ratio ((g/g)%)Surface-crosslinked Example 2 42 36 25 1,050 water-absorbent resin (2)Surface-crosslinked Example 3 34 35 21 714 water-absorbent resin (3)Surface-crosslinked Example 5 25 26 18 450 water-absorbent resin (5)Surface-crosslinked Example 6 23 24 25 575 water-absorbent resin (6)Comparative Comparative 28 26 73 2,044 surface-crosslinked Example 1water-absorbent resin (1) Comparative base Comparative 48 — 41 1,968polymer (7) Example 7 Comparative Comparative 33 30 49 1,617surface-crosslinked Example 7 water-absorbent resin (7) Comparative baseComparative 39 — 60 2,340 polymer (8) Example 8 Comparative Comparative28 28 65 1,820 surface-crosslinked Example 8 water-absorbent resin (8)SXM-75 Stockhausen 32 29 43 1,376 SXM-77 Stockhausen 31 25 62 1,922ASAP2300 BASF 31 26 68 2,108 ASAP2300 BASF 24 24 84 2,016 Drytech Dow 3031 60 1,800

Various details of the invention may be changed without departing fromits spirit not its scope. Furthermore, the foregoing description of thepreferred embodiments according to the present invention is provided forthe purpose of illustration only, and not for the purpose of limitingthe invention as defined by the appended claims and their equivalents.

1. A process for producing a water-absorbent resin, which comprises thestep of polymerizing an aqueous solution of water-absorbentresin-forming monomers including acrylic acid and/or its salt as majorcomponents, wherein: (1) the polymerization initiation temperature isnot lower than 50° C.; (2) the solid component concentration in ahydropolymer as formed by the polymerization is 50 to 73 weight %, and(3) the polymerization time is shorter than 3 minutes.
 2. The process ofclaim 1, wherein said solid content of said hydropolymer is 60 weight %to 73 weight %.